The idea behind setting the preload first is to eliminate the drag of the rest of the gears and bearings from affecting your readings. Given the preload is so low, I wouldn't worry about it, just zero preload + plus a bit. I recently did my Lotus Seven's diff as I fit a torsen unit. There you measure case spread. You tighten the adjuster nuts, keeping the proper freeplay, and spread the case by 12 thou. That's quite a bit more than what the Europa gets as its cases are so much stiffer and the bearings much larger. All that to say, if you get slightly too much preload, it is no big deal.
The races should be a tight fit into the adjuster nuts, not a push fit. If you feel they might be not tight enough, use retaining compound.
If you have kept it clean and covered, there is no need to wash the diff assembly. If it dirty such that you feel it needs cleaning, disassemble it and do it properly.