Before install freezer xtreme cpu fan should i clean off existing thermal grease

Yes, I would do so to ensure that it seeds evenly across the processor. I'd recommend using rubbing alcohol (at least that's what I have always used) and a few Q-tips.

Yes but i recommend being extra-careful. Remember that any static could end your cpu's life, and any damage on the underneath connectors could make it unusable. Also avoid having grease ending up on sides or under the cpu, obviously.

I have not tried with alcohol yet, I usually just grav the softest tissue i can find arround.