This is to suggest that Austen's depiction of Henry at Portsmouth is deliberately enigmatic. The procedure is simple: Austen keeps Henry's thoughts back from us; we watch him, we listen to him, but he is consistently seen from the outside. He is allowed to write no letters of his own (so no self-revelation there); he is filtered through the consciousness of Mary or Fanny; we get a feel for his upper class presence through Fanny's father's response to him. But what he thinks or feels we are not told. We are given no access into his mind. The narrator is for once silent.
What are his words and actions? Well, he is exquisitely tactful and considerate, appears not to notice the slattern and desperate shifts of the genteel poverty he finds Fanny in, though Fanny notes--and with a relief we are expected to infer his understanding of her shame from--he does not stay to any meals. Every word, or gesture, or act he makes seems intended to make Fanny comfortable: it is he who makes sure she and Susan get to shop, he who walks slowly, he who provides an arm for a long healthful walk on the docks. Another clever device here is the presence of Susan--Austen prevents her Prince Charming from even talking of love, and since we have heard Henry's insinuating tones in this way before, the lack of such talk works to make us respect him more, take him as more sincere. What he does talk of shows his ability to discern the machinations of a self-serving steward and the vulnerabilty of the poor who pay their rent to the steward and are therefore kept at arms' length from their landlord.
Is this distancing of Henry from us anything new? In part yes. In the earlier parts of the novel Austen was concerned not to allow us to fall for his allure--as readers sometimes fall for the glamorous and spontaneous, half self-deluded deceit and selfishness of Willoughby in the early sequences of Sense and Sensibility. So we never see him rehearsing with Maria; we hear of it, but don't see it. He is kept at a distance from us in that we don't enter into his consciousness, except when he is talking to Mary Crawford. But there we are allowed some access: if we are to remember he is talking to his sister and partly for effect, still he is reasonably candid with his sister (who is not at all worried over the Bertram girls and not very much over Fanny). And what do we discover: well with respect to his plans to make holes in the heart of a dependent naive girl who has had the temerity not to make up to him, and admire him, a repellent desire to take a curious revenge by conquering her despite herself, and a joy in triumph.
I would say therefore that the portrait of Henry is consistent throughout the novel. Nowhere in Volume III do we know for sure of anything in Henry's mind which is inconsistent with the earlier Henry. We are simply left to believe that Henry has changed his outer ways because, as he says, he takes Fanny to be "different" from all other women he has met, and, as the narrator adds (the narrator is not silent on Henry in Volumes I & II), his appetite and pride and vanity have been touched to the quick by Fanny's implicit snubbing of him.
Why this partial change? I suggest Austen meant slowly to make us take a more favorable view of Henry, to experience with Fanny a gradual trust in him, without ever being quite sure that the behavior will be permanent or is the result of any determination to habituate himself to living differently. She wants on the one hand, to make us infer possibilities of real decency, kindness, gentleness, responsible behavior towards others to come, but on the other, not to make such an absurd reformation in her villain-hero as earlier sentimental dramas did (where in the last scene of the last act the villain suddenly weeps and reforms and falls into the arms of his sobbing virtuous wife--this happens in a play called Love's Last Shift). It must be believable, within the limits of probability.
Let me stress here that the point is to make us see what is good in Henry, and, as I believe we talked about in Volume II, Henry as chivalric gentleman at Portsmouth, a kind of Colonel Brandon without his flannel waistcoat, is slowly prepared for in Volume II. We are there, as we said, allowed to enjoy ourselves in his company mightily at cards (as opposed to the discomfort and sleaze of his behavior at Sotherton). We see how talented he is. He speaks eloquently of his change of heart in the case of Fanny (not all women, just Fanny who he sees as something different, which I suggest the narrator means us to know is absurd, for the narrator has not idealized or sentimentalized Fanny as Henry now does), though I'd like to point to one suddenly heart-felt remark of Henry which all his supporters have overlooked and is perhaps one of his most disinterested and spontaneous and therefore sincere comment in the novel. When he returns to Mansfield and asks after "Rushworth and his fair bride" and "Julia," and listens to Mary's light cool tones of mockery as she moves from figure to figure, he answers with a sudden rush of earnest feeling for once, almost as if he is surprized into revealing an element of real feeling for Maria:
"Poor Rushworth and his two-and-forty speeches!" continued Crawford. "Nobody can ever forget them. Poor fellow! I see him now--his toil and his despair. Well, I am much mistaken if his lovely Maria will ever want him to make two-and-forty speeches to her"; adding, with a momentary seriousness, "She is too good for him-- much too good" (Chapman II:5:224; Penguin 23:235).
There is the core of feeling Maria touches when she leaves her husband's house and appeals to his better self to be true to her. We have seen that Austen shows more sympathy towards Maria than is usually granted; I know that she suggests his running away with Maria is a throwing away of a woman he could have been permanently happy with and loved (Fanny) for a woman he doesn't really love and cannnot be permanently happy with, and also know that nowhere does she condone sexual encounter or living together outside marriage. But I'd like to suggest she is that true to human experience that we can say Henry's noblest moment in the book is when he answers Maria's deep need of him at the moment she has thrown her security to the winds. I think he rises to that occasion, and I suggest the button Maria could press is in the phrase: "She is too good for him--much too good."
The final and by no means least important reason Austen keeps Henry's behavior enigmatic at Portsmouth is she is working the element of suspense in this final volume for all it's worth. We are not let into the recesses of Henry's heart so that we will keep turning those pages, and read on to see what will happen next. We can't tell. Here is the wealthy scion, spoiled darling of the tiny upper class of England, debonair, witty, suave, and he's willing to sit in a tiny dirty room with the boring nobody Mrs Price. Can he keep it up? Is he really serious? Will he really marry Fanny and take her to Evringham where he can read aloud to her and she explain to him the constellations? Will Fanny really get to take Susan with her?
On the first read of the letters the element of suspense is strong too. As with Emma only the second reading of the letters reveals to us the complicated intrigues and emotions that have been forming a story out of sight; on the first reading, we are with Fanny. My God, what can Mary mean? What has happened? And then Edmund's letter where he says he has not the heart to tell all. Fanny must wait. So must we.
And the suspense element is added to by Fanny's loneliness and outcast state. She doesn't know what's happening. Lady Bertram's letters are a cloud of words which issue from her longing for Fanny but tell nothing objective. What is happening to Tom? How is he really? She longs so intensely to return to the circles from which she has been excluded. She sits and remembers Cowper's lines about how a schoolboy longed to return home. We ask ourselves, is Sir Thomas determined to leave her there until she makes some sign of accepting Henry? Or was it just coincidence that he takes her back when Henry is no longer an available candidate for her hand. I know I have offered a sympathetic interpretation of Sir Thomas's behavior, but there is a hard-nosed one too. As with Henry, Austen just gives us Sir Thomas's behavior and his words which are shaped by the social situation he speaks in. As in life, we can never know for sure what he had in his mind. If he or Henry were real, I'd say they weren't sure what they had in mind. Which is just what Austen wanted me to think.