Is the account level only valid for 1 year?

No, but I should. I just have a lot of free time :)

Sometimes I will look up a previous comment I made by searching through my messages for something someone replied to me, but I usually just type a new message.

Yeah that does happen, but it's obviously a bit rare. Mostly it happens by mistake I think, I remember the Daedalic bundle causes a temporary bump in CV for all the included games when it goes on sale.

Yes, anytime the base price of the game is changed in either direction, anyone who created or won a giveaway for that game should have a change in CV.

I don't know how often the site checks for price changes, but I would imagine it is fairly often because when certain games are bundled in sales that temporarily change the base price, it messes up the CV and those CV changes happen pretty quick. This happens a lot with games from Daedalic entertainment. When they are running their bundle sales on Steam, people here that previously gave away any games in the bundle get like $100 or $130 CV per game. These games have been bundled so many times that a lot of people have given many copies of the games and it causes a drastic change in CV until the sale ends. Users see their level go up and then when the sale ends, they drop back down a level and create threads asking why their level dropped.