The question would be whether Cohen is a credible witness and whether that
would be enough to convict Trump.
That is an important element, of course.
If it were just "Cohen says X-Y-Z" then yeah, credibility is an issue. But if it's "Cohen says X-Y-Z" and you have a tape of X, electronic documents of Y, and a paper trail of Z, the tends to establish credibility.
It would be very easy to suggest that Cohen made that statement as part
of a plea deal
to reduce his sentence for the tax-related crimes he's
actually been convicted of.
Yes, it would be, but the problem I have with that is
-- Cohen was not arrested or arraigned - he walked into the FBI office and confessed. Why "make up stories" about something you did not do, when you haven't even been charged with anything yet?
-- It's one thing to make a plea deal and say "I did [lesser thing] X instead of [bigger thing] Y". It's quite another to implicate someone else in the crime. If you implicate someone else, and your story is easily falsifiable, you've just blown your plea deal. Prosecutors aren't stupid. If you name names, your story better be independently verifiable. Lying to the FBI to try to get yourself a reduced sentence isn't going to work.