Playing Gurmag Angler instead of Tombstalker could be right. The need of UU for TNN and BB for Tomstalker indeed puts a stretch on the mana base. Flying is pretty good though, so I'll keep Tombstalker for now until i've got more test results. A friend of mine suggested a 1/1 split of Stalker and Angler. Maybe that's the way to go. Without the Tombstalkers, the Bayou isn't needed.

Regarding Hymn: I honestly don't like the BB casting cost. The main thing i disliked about traditional Team America builds in the past was the extremely cluttered CMC 2 slot. The amount of CMC 2 interaction just feels sooo clunky. I never managed to put up decent results with that variant. But since other players are successful with the deck, it's probably my own fault ;)
I also prefer the information Thoughtseize gives me over the potential Hymn blowouts.

Leovold: I only tested him for a short time, so my results may be off. I don't like the card in Delver shells. I prefer my creatures to either demand an immediate answer (like TNN or Tombstalker) or to generate an immediate effect (like Snapcaster). Leovold doesn't help when we are behind on the board, like Snapcaster -> Removal or TNN. Of course T2 Leovold can be a beating, but I think decks like Reid Dukes GP list are way better at playing that game.

In a couple of games, Leovold was the final nail in the coffin when I had out other threats and my opponent couldn't cantrip into answers, but I probably would have won those games anyway and Leovold felt like win more. As the only threat on the board, he was just a 3/3 for 3 that instantly died and drew me a card, which isn't impressive. I even prefer Snapcaster into Ponder in that situation.
I really like the card in grindy midrange shells though.
But feel free to test him. Maybe you get better results ;)