No problem.

The mixing of modular signals and ipad audio will be taken care of via whatever ipad app you're using (there are plenty of options). I found AUM to be really useful, as it lets you route audio in and out of different apps to ultimately be sent back out of the ES-8. Sure, you could just stick with Logic, but AUM will give you a lot more options.

The purpose of the output module would be to combine two mono outputs of the ES-8, for example outs 1+2, into one trs out with suitable levels for headphones.

Important distinction between 'combining' and 'mixing' here. The output module in theory shouldn't be doing any mixing. rather it's providing you with the TRS jack your headphones need.