Answer 1

Apparently placing the goal flag object too close to spikes, even when it's triggered by the destruction of dust, causes players to repeatedly die and respawn on the results screen. Oops.

Answer 2

Just make ending platforms under your goal flags, the flag attach range is long enough that the ending platforms don't even need to be in the same screen as the end of the map.  Have you played any Atlas maps?

Answer 3

Not many, no! Sort of figuring things out as I go along. Thank you for the advice, though.

Answer 4

It's much easier to make maps the more maps you play and the more you SS the base game.