does a player have to encounter the tinker imp ,,,or can he destroy the imp without having to have one of his items moved on a die roll????

NOPE

I don't know, it's one of those need clarification questions. TI reads "before engaging in psychic combat", but Cross (and Priest) allow you to destroy Spirits even without resorting to psychic combat.

I'd House-Rule that Spirit-destroying effects get in action before any rule on the Spirit card is resolved.

i think by destroying the tinker imp, i have avoided the encounter, and it needs to be encountered in order to use its ability

I would think that you use the cross as soon as you encounter the TI, so he dies in a poof of smoke wailing "but... but... but... I wanted to tinker with your items!"