Severus & Gilderoy: a rhapsody
I adore Snape/Lockhart fics, and this is one of the best!
Your stealth de-aging/re-aging of Severus is a brilliant way to get them together. It certainly explains one bit of how Lockhart could come to trust Snape, or even simply to pay attention to him. (UST is clearly another bit - yayy for SS/GL UST!)
In turn, Snape's sussing out Lockhart's ploy is another brilliant and creative move. Each of them "has something on" the other, each is bound to admire the other for that cunning - and of course, gain self-confidence (or self-flattery, lol) because of his own cleverness. Which in turn makes them like the other even more. It's a giant circle of mutual- and self-admiration fed by their mutual love of being cleverer than everyone else. A very Slytherin match-up!
A con artist and a spy would surely have so much in common, after all. I can imagine this pair carrying out all kinds of schemes as they go off into their post-war future together. One of the first, of course, would involve guilt-tripping Hero Harry Potter as Snape suggests, into (at the least) a decent war pension for these two anti-heroes.
(I can imagine them having to thread the narrow eye of a suspicious Hermione in the process. And incur the equally suspicious eye of Lucius Malfoy, who surely sees himself as the Wizard Most Entitled to Everything. He'd jealously come calling, trying to figure out how a faded fake celebrity and a certified spy connived their way into Harry's public friendship and a handsome Ministry stipend.)
But this romance is not only an affair of mutual self-interest and scorn for the wizarding world. The undertone of insecurity in Gilderoy here, especially, is poignant and deft. I like how you just touch on it lightly, a hint here and there: The way he "wistfully" notes how Alice and Frank have family, who visit them, even though he wouldn't want to be like them really. His hands twisting the satin of his dressing gown (the lilac one that usually made him feel as if he could handle anything but which only left him feeling underdressed for this meeting). The way he offers, then pleads to help Severus write whatever-it-is he's writing, with his "joined-up writing." The Snape who hides his inner fears and shyness is often written, and I love seeing how Gilderoy's matching form of insecurity can be a cure; how each of their unique contours of damage and fears, brilliance and skills, can come together into a perfect fit, making them together a powerful whole.
It's a lovely H/C you've created here for these two. No wonder I like them so much - with their cleverness, they each have a tremendous capacity for understanding other people's weaknesses. Usually that's only seen for its ability to exploit people, or undermine oneself. But in this fic, instead, you've nurtured its potential to burgeon into a beautiful, empowering empathy. These are two of the most compelling HP characters, that were discarded when they still have so much to offer. What a lovely fic this is, bringing it to life.