Since true name nemesis gains protection from a player, can that play force an opponent to sacrifice it using spells like geths verdict or celestrial flare? and can board affecting cards like wrath of god or supreme verdict affect it? or do they not work because the player it has protection from controls those spells?

from my understanding of protection rules,

the protection would be in effect only as long as the nemesis is being targeted, this would be bypassed by targeting the player controlling Nemesis instead.

any indiscriminate sweepers and effects that would force an opponent to sacrifice a creature (assuming they would pick the nemesis) would be able to get rid of it.