It doesn't sound like it was exactly the same question.

The OP was asking about 20 pounds split between a saddle mounted bag and a large handlebar bag. So probably 12-15 pounds in the back and maybe less depending on how much is in the handlebar bag. That is much less than your proposed 20-25 pounds in 2 panniers.

He made no mention of using a rack or panniers on the back, or the front for that matter.

As far as the trailer goes... Not what the OP was asking about, but lots of folks have done long tours with trailers on similar CF bikes without incident.

I do agree that it is probably a bad idea to add stress to a carbon seat post and would probably use a aluminum one.