Not true about VC. Since the VC acts like 1 switch, a multi-interface LAG is created from [in general] different members of the VC and all are active members of a single LAG. Think of this as a chassis with multiple modules where you create a LAG than spans across the modules. All interfaces are active, so you have link level redundancy and HW module failure redundancy. You VC LAG will be all active members so you'll get 2 x 10G throughput.
LAG will use a similar algorithum as MC-LAG as how to pick link on which traffic is sent.
I doubt there is a specific document for MC-LAG for QFX3500, but there should be a generic MC-LAG doc using some product, for which the configuration/set-up should be the same. One concern maybe that QFX3500 does not support any current code, so some of the newer MC-LAG capabilities and improvements may likely not be there. I've never done MC-LAG with QFX3500, so I might suggest VC might be a better and safer option.