1. Tom divorced Mary last year (no is/was) OR 1. Tom was divorced from Mary last year (add from) 2. Gandhi was committed to nonviolence. He believed in it all of his life (was: Gandi is dead, so it is past tense since he cannot be committed to nonviolence anymore)

In both examples the past tense is correct and, as such, the verb "was" would be used.

1) Neither. Tom divorced Mary last year. Tom got divorced from Mary last year. 2) Grammatically both 'is' and 'was' are correct. But 'is' would only be used if Gandhi were still alive (or if we were imagining a situation where he was still alive.) Given he has been dead for decades the sentence you are looking for is: Gandhi was committed to non-violence. He believed in it all his life.