I have an L1, and it is quite strong, but they are flow pumps, not pressure pumps, so probably dont do great against a lot of head pressure (which i dont think you will have here with your return scheme). Also, my L1 runs at 75% and it is barely warm to the touch (I run it externally).
Just make sure that you plumb them so that the intake/output are max size (not using the ecotech supplied blue pieces).
Full disclosure, I am on my second L1, the first one nuked itself after 7 months, ecotech overnighted me a replacement (the housing around the impeller melted and bubbled and started to rub on the impeller, yikes). But keep im mind you can buy like 4 backup pumps for the cost of an Abyzz...
Honestly, I think that your plan is great, L1 for return, and M1 for manifold. That way your return flow doesnt change every time you tweak something on your manifold, which will save you from constantly having to tune your drain lines to maintain a quiet drain.
Oh, also, you can calibrate the L1 for return, so that its max and min flows are actually useful (min flow barely runs anything to the tank, max flow doesnt overcome your overflows).