That's about all there is to the charging system. The stator puts out AC, the R/R rectifies it to DC and regulates it down to ~12V. If you're replacing it anyway, I suggest looking into the
MOSFET R/R mod.
I would expect it to have problems at 8V, both in actually cranking, and in working voltage for the igniter. Use your ammeter and see if you're getting any usual current draw while the bike is completely off, or key-on but not running.
Is 45VAC within spec? On the 500, they say 60. (The 250 may very well be a different number, but they use the same R/R.)
My Clymer's R/R test procedure says that you should get a high reading one way and a low reading the other way (at least 10x difference), but it doesn't say which direction you should get each. I'm thinking that your test may not have been conclusive of a bad R/R if the only issue was opposite directions.