1. Freehub is a much better design than freewheel, and is a more practical choice (cassettes are cheaper and easier to replace than freewheels).

2. 4mm less OLD puts the hub flanges 2mm further to the left than a 130mm OLD, and any OCR's spoke holes are offset to the left more than this. So if you use an OCR, you can have an 11spd/126mm OLD with less dish than a 7spd/130mm OLD wheel.

3. Sticking with the 126mm spacing avoids straining your stays and pushing your dropouts slightly out of parallel, causing your spindle to bend slightly when clamped in. OCR is the solution.

4. If you want to put the wheel in a later frame, you can probably keep the spindle (3mm of spindle in the dropout should be enough), and just add a 4mm spacer behind the left locknut, and tighten all the left spokes a bit.