Ah, okay. Those make much more sense now. Extrapolating from that, a dwarf with AG -5 wouldn't be able to have more than 1 rank in any of the AG-based M&M skills, and a hobbit with ST -20 wouldn't be able have any skill ranks in Chain or Plate (unless a ST limit doesn't apply). I never caught any of that before in all the years I played.
The Perception SM sounds reasonable, too. I imagine it's only applied in pursuit or hue & cry type situation (basically, when the party is actively stating that they're trying to hide).
On the topic of MM skills, those were some of the first things we always house-ruled, since it seemed unsatisfying that a power-gamed PC could have a chain MM score that was higher than the soft leather MM score. We tried all sorts of things, including:
1. Combining them down to only three MM skills: none, soft armour types (leather or chain) and rigid armour types (leather or plate).
2. No add ranks can be added to one MM skill until the previous skill is maxed out.
3. Each rank purchased adds 1 rank to all MM skills, but the costs are stacked like normal skills (1:1 rank, 3:2 ranks, 5:3 ranks), and the cost is doubled (that makes 30 pts. to max out all MM skills as quickly as possible, which isn't too far from the 26 pts. it would take normally).
I don't think I ever found a house rule that seemed really satisfying though, and honestly I doubt the rules as written would have caused much of an issue in play 99% of the time. When I run MERP again this year I'm tempted to just stick to the rulebook and see how well they run in practice.