The first part of Undercity Uprising’s effect affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t gain deathtouch.

If both target creatures are illegal targets by the time Undercity Uprising tries to resolve, the spell doesn’t resolve. No creatures deal damage or gain deathtouch.

Remember that a source with deathtouch has to deal damage, but not necessarily combat damage, to destroy another creature. As long as the fighting creature you control has power greater than 0 (and the damage isn’t prevented), the fighting creature you don’t control will be destroyed.

If one of the two target creatures is an illegal target by the time Undercity Uprising tries to resolve, no creature deals or is dealt damage. Creatures you control still gain deathtouch.