Hey RG, the line input jack is balanced if you want to use it, just make a lead XLR female to stereo jack (wiring is on page 10 of the manual), or buy a XLR > Stereo jack adapter, they're not expensive. You can plug into the mic input, it's just not ideal gain structure pumping a line level signal into a mic input (kind of going backwards from a gain point of view), sure you can turn the Rockmaster output down and the mic channel gain right down and make it work, but just use a XLR > st Jack adapter and use the line input, keeps it balanced and much better (correct) gain structure.
The MP1 Fx send always has a signal (comes just after the tube > Eq section but before the chorus). Taking a feed from there won't break the serial chain, the loop on/off in the MP1 patch toggles the Fx return jack path. You can do the same with the Rockmaster but use a Y lead so the dry signal is fed back to the return and also to the Fx input so you don't break the serial chain.
You'd only need a cab sim if you are going direct to a desk and full range speakers and you want to use the same patch as you use with cabs. A patch adjusted for cabs will have/need much more tops than a patch tweaked for full range as the live cabs are 12" speakers and don't (usually) have a horn tweeter so they (the speakers) roll off the top end, thus we boost the tops so it sounds right.
(balanced output provides 600ohm, transformer balanced signal to be used as "direct" patch into mixing consoles, tape recorders, etc. The signal at this point has been frequency compensated for low noise operation.)I read this as they have reduced the tops to compensate, which is the main thing all cab sims do and also what the MP1 does on its headphone out for the same reason. (BTW you can use the MP1 headphone out direct to a desk etc for recording). So using the line out XLR and going to cabs (as is what you want) you'll have less tops and have to compensate in your patch (or IIRC correctly you use an Eq before the amp ?). If you just use the Rockmaster L/R outs into ch 2/3, Y lead to split off the Fx send and Fx L/R outs into ch 4/5, you now have parallel Fx and maintain stereo.
Anyway, try it all these different ways and see/hear what sounds best for you.