Hey El, there's so many ways to patch things and even if they aren't "optimal" it helps your understanding
so always worth a try.
When I first got MP1 I combined it by using headphone out to rockman XL100 headphone in and rockman headphone out to 2 Marshalls (with resistors to lower the voltage).
The MP1 headphone out is after everything like the main outs. I'm not sure if it's tweaked to not be too toppy ? as the idea is you can listen to your MP1 without disturbing anyone (bedroom etc). So it has loop + chorus etc, sound is (much) the same as A/B outs. It's designed to run headphones @ 600ohms or more, the returns on the Lex should be ??
k ohms so should be no problem there. IIRC there is a headphone out level control ? which should be independent of the main (front panel) output level which controls A/B out levels (but need to check that). So to get the right gain/balance into the Lex returns use this (hp out) level control and Lex return level settings (pick +4db on Lex). And yes a TRS > TS/TS to connect.
To put Rane in MP1 loop, just use mono TS > TS jack leads into/out of the Rane jack sockets (leave the XLRs out) and adjust for unity gain (no overall volume change from loop in or loop out), use MP1 loop level control and then Rane in/out level controls (which I assume are programmable per Rane patch) to get right for each Rane eq patch.
If you have 4 inputs on your mini mixer, you can have say MP1 A/B outs > chan 1/2 and Lex L/R outs > chan 3/4 and balance how you want. If you adjust the MP1 and Lex patches you can have any balance of the 2. Say you want MP1 with Lex FX but no Lex guitar sounds, make a Lex patch that just has FX and guitar sounds = 0, if you just want Lex (no MP1) make MP1 patch all 0 (just MP1 master set to 0 will do this but best 0 everything so less noise). So that's the 2 extreems, any balance in between can be dialed up with different patches (so do patch pairs to get what you want, remember MP1 goes 1 - 128 = 0 - 127 in many other gadgets so MP1 patch 100 may = Lex patch 99 ?? etc)
Of course you have to split the guitar signal between Lex and MP1 (obviously) for above.