So the Bitcoin security assumption (most hash power is honest) is not strong enough to make ripple secure if translated to comparable terms ('most trusted nodes in the system are honest').
Your analysis is correct. In degenerate cases (small numbers of nodes, sparse trust) the topology works against you as much as the number of colluders. With larger numbers of nodes, the topology works in your favor -- the more nodes there are, the more conspiring nodes required. The cost to acquire a conspiring node may go down with the number of existing nodes, but not linearly.
How do your cryptographic signatures that show if someone misbehaved distinguish between them misbehaving vs trusting someone who misbehaved?
There is no distinction. If you mismanage your trust, you have failed. It is not so much a moral judgment but more a "this isn't working out" kind of thing.
Couldn't I protect my reputation by attacking by simply arranging to trust dishonest sockpuppet nodes?
That would cause you to validate the wrong things.
If I can't then isn't there considerable pressure to only trust the same nodes everyone else trusts?
Yes, exactly. So long as there is agreement, there is no issue. Every honest participant prioritizes agreement over everything but following the rules. (The bitcoin analogy would be that priority one is that blocks are valid. Priority two is that you pick the longest chain.)
The advantage of our scheme is that you get to choose who to trust. The disadvantage is that you have to choose who to trust.