Good questions!
1 - Play with a starting Momentum of 2. This will prevent any weirdness from happening with the other rules.
2 - I'll update the Spades rule to represent a roll+Momentum total of 3 to 9 (to account for the potential to crash).
3 - Each route is 5 cards (found under the "How to Play" section). So playing cards in reverse would mean drawing 2 (when possible) and playing the second card drawn before the first. It's just a bit of fun that could provide a possible twist of fortune.