As i fiddeled a bit with magnetoes I think that i can trow in a few bits. In some cases you overlook the most obvious, no offence I have been there several times myself!
The first thing is in wich direction is full advance? Is it with slack wire or with the lever pulled back? In many cases it’s with slack wire on BTH and with lever pulled back on Lucas. But if the magneto has been converted to run the opposite way around it could be the other way. And it can been “born” that way, too.
Look from the breaker side of the magneto. If the cam rings moves, when you pull the lever back, in the same direction as the breaker plate rotates. Then you’ll have full advance with slack wire, and opposite.
Regarding weaker sparks on retard: All manual magnetos are that way. The internal timing is at its best at full advance.
(But on the automatic advance magnetos the armature turn, so they’re always in the best spark position.) The most awkward work for a magneto is slow speed at full retard. If it don’t miss a spark there, it’s probably OK.