Is there a way to limit players to spawn in certain locations or certain planets? Moreover, is there a way to change the distribution of metal so that it's not as obvious where the spawn locations are? I would find it interesting to do a 1v1, players spawn at different planets game but I can't see a way to force the players to spawn on different planets.
Not currently, no. Although a multi-planet spawn 1v1 isn't that great. More often than not it just devolves into nuke tossing or using a KBW.
I just had a 10p FFA with 5 planets and 5 of us spawned on one 200 radius planet. I am waiting eagerly for some sort of way to control spawns per planet.
Fiddling with the metal quantity and metal density sliders can get some unique results. If you lower the density, then metal won't be in as many clumps.
