How does Mormanism explain the other religions?
They are all based off of a set of beliefs just like we are. A lot of them have come from good foundations and have a lot of good taught, they just don't have ALL the teachings that we need in this life, or the Priesthood.
If Mormonism is all volunteer and about serving, why are the temples so grandious and splendid instead of a building that serves the purpose without being austentatious, giving the remainder of the money to charity?
I'm not sure if I could answer this question to be honest. The best way for me to say it is they are the House of the Lord, Heaven is so much more beautiful than anything we could imagine on this earth, and the temples are meant to remind us of Heaven and Eternity and what's beyond this life.