But a few things are worth noting:

a) The attack on the colonies was not expected to be nearly that easy. Six was named a Hero of the Cylon because they judged her efforts as the lynchpin in making it work. Without her, it would have been a longer, protracted war.

b) For the S7 Cylons, they were in many places, such as the Gina that ended up on Pegasus. I bet 6 other battlestars had a Cylon on board in the military, and probably many civilian ships had them as well (where the duplication woudl not be so obvious.) However, you can bet there was not another Tigh in the military (at least not near his age) as he would be famous enough as a Colonel to not be somebody you duplicate. Anders was also famous. But you could duplicate them at different apparent ages.

c) We don't know who planted the transponder. Could have been Tigh (unconsciously.)

e) Destroying the Galactica at New Caprica doesn't destroy the human race, just its main pesky military threat. Throughout all the attacks on the fleet, they have aimed almost entirely at the military ships, even though civilian ships would be easy for them to nuke -- especially if killing all humans is their goal.