CarnivalOn play effect is really weak (compare with Candlestick Maker). The reaction is extremely political, which goes against Dominion design philosophy (though if you like that kind of card, by all means try it). You can also use the reaction on yourself, but that would just be a restrictive version of Workshop. As it is, I'd start testing at $2.
But again, the political aspect of it is not great. Like, imagine it's a 4 player game. I play Witch. Player to my left gains the first Curse, then I and other players all discard Carnival. Maybe some even discards 2. That guy gets 4+ Curses all at once. Maybe the pile was close to emptying as well and nobody else has to take a curse. Not fun.
I'd try having it just gain
you a copy of the card. Then you could use it as a restrictive workshop or alternatively a mini-Smugglers that can target more players (other than just the one to your right).
TillYes it should have the attack typing. The wording could be tighter:
Trash 2 cards from your hand. Choose a card in the supply with cost exactly equal to the total cost in coins of the trashed cards. Either you gain a copy of it or each other player does, your choice.
The main change here is that now you don't have to choose a card for each player. This removes some of the politics and also speeds up action resolution. Seems alright. It should play a lot like Ambassador early except that it's harder to get and it's going to drain the Estates pile. Later on, you can use it as a mini-Forge.
The on-buy effect looks fine for balance, but it makes the card as a whole too complex. I'm not sure it will all comfortably fit on one card. If you do keep it, I don't see a good reason to forbid Till from gaining itself. You can't empty the entire pile since it is an on-buy effect, not on-gain.
ParadeDefinitely not going to fit on a card. Take a look at the hybrid VP cards and note how the VP shield already takes up half the available space. Also note that it goes below the line, so adding an overpay as well would be kind of ugly even if it could fit. But it really won't.
Card space aside, I don't know how to feel about the on play. I'd like it better if the Estate gain was mandatory. I don't like the overpay. Seems too dangerously powerful to me.