John, I'll add a few comments. On 3), yes just keep the sub crossover disabled and let your 1802 handle it without interference from the internal sub crossover. As you indicate in 2), if you also use the internal sub crossover the 1802 crossover can't possibly know that, so they might interfere with each other at certain settings.
On 1),if you set the fronts "large" and you have the sub mode on your 1802 at the usual "Norm" setting, the sub would get none of the low bass from the fronts; the fronts would handle it all(and yes, since the M60s have good bass, there might be relatively little difference if the program material doesn't have a lot of very low bass). The sub would get the below 80Hz bass from the center and surrounds if they were set small and if there was any special low frequency effects(.1,LFE)in movies it would also get that during the times that it occurred. If the fronts were set "large" and you set your 1802 sub mode to "+Main" then the sub would also get the below 80Hz bass from the fronts simultaneously with them, not necessarily a good idea.
