It surprised me, because I too had this vague idea that Gellert had died shortly after Gregorovitch, in August 1997 - about two months after Albus. But just after I read this story I was checking something about the sojourn at Shell Cottage and discovered that Gellert in fact died while the Trio were en route to Malfoy manor, on 30th or 31st March 1998.
It's a damned nuisance - you can't cut Gellert out because you write him, and his effect on Albus, so well, and he deserves to be in there.
You do write canon absolutely perfectly, apart from making Snape gay. I know Snarry is your thing but I kind-of wish you'd do some stories with a het Snape so we can see *perfect* canon. If it weren't for the AU-ness of making Snape gay one could say that this story absolutely *was* what happened in the background to canon, but then it can't be canon, because it's got gay!Snape.
How do we know Snape is straight? Because according to my bi friend gay men are always *looking*, even if it's just window-shopping, so if Snape were gay there ought to be some indication that he was assessing the beautiful Lockhart sexually (even if just to think "Thanks but no thanks") and had some kind of a charged reaction to being embraced by Slughorn. Instead of which, there's no sign that he is aware of Lockhart as anything other than an annoying pillock, or has any reaction to Horace except "This si my friend: my friend is very drunk", and there *is* a kind of frisson in his scene with Narcissa.
Harry, conversely, is definitely bisexual, because he is very aware of how other males look and he assesses them in terms of their attractiveness. He's not vain himself - it's not that he's thinking "Is this guy more or less attractive than me?" - so it must be that he is indeed "looking". We know he fancies girls as well so he's bi, not gay - but bi with the gay component quite strongly marked. JK even jokes about it by having Harry joke about it, saying Ron isn't his type.
You could conceivably get away with a canon Snape who was slightly bi tending heavily towards straight, but was persuaded to make an exception in Harry's case because he has Lily's eyes....
That aside, even your version of the afterlife/astral is convincing: many psychics report that there's some kind of channel which enables the dead to look at the living and which functions like a device esternal to the user, rather than just a method. It can be any kind of viewing device the user can visualise strongly, so it would be perfectly possible for Lily to conceptualise it as a TV set and it would then look that way to her.