You're right. Because I have been able to start my M25 on a 90A MRBF, it does not mean that I am suggesting that it should
be done. If I catch that bone-head skipper doing this again, he's fired!
However, I do recall reading somewhere that the starter on an M25 should be able to do it's job with <100A. If it is sucking more juice than 135A, I want to know about it, because it means that the starter is acting up. Also I have a spare 150A fuse that I keep - just in case.
Your second point, I believe, is the theory of fusing a circuit; the fuse protects the wire, not the device
. For details see; ABYC Wiring Standards
All good stuff!