Can't give Snow landwalk, because Snow is supertype, not a subtype.

Can only give landwalk of land sub-types that exist (so you can no longer give "Island of Wak-Wak walk").

Paying cumulative upkeep is always optional. If it's not paid, the permanent with cumulative upkeep is sacrificed. Partial payments of the total cumulative upkeep cost can't be made. For example, if a permanent with "cumulative upkeep " has three age counters on it when its cumulative upkeep ability triggers, it gets another age counter and then its controller chooses to either pay or sacrifice the permanent.