To cast a fused split spell, pay both of its mana costs
Put into actual numbers, each Bill you activate costs you 2-3 turns in the drawn-out games Base-Fossil produces: the 2 cards drawn from Bill, and the Bill card itself leaving your hand, which you could have shuffled back into the deck with Lass or Gambler for an extra turn of draw to help you take your last remaining Prizes or deck your opponent
The exiled card of Expressive Iteration may often be a non-optimal play and requires either a very low curve to fully capitalize on it exactly like Grixis Shadow or UR Murktide, or an ongoing finding of additional land drops
Probably not, though there's no guarantee of that, admittedly
Playing a land from the top of your library counts as your land play for the turn
Cleaning the edges is definitely a little time consuming, but to me its literally one of the most satisfying parts of the job