Hi Rolleiman (do you have a name we can call you by?)
I really do think you're getting bogged down with the flare 'problem'. As you say you used Nikons professionally for years, I think the whole rangefinder thing will be a bigger shock to the system than possibly occasional flare in the viewfinder. I use Leicas because I find them very convienient to use, not awkward at all, and am happy with the lens range available: mine are all modest aperture versions - no speed giants!
Although I still have my M2 and lenses (which I bought from its original owner - a retired Major in 43rgt Royal Marines Commando!) now my eyesight is so poor close up, I find the LED meter in the M6 so much easier to use. I cannot read the MR meter on the M2 without putting my glasses on. Don't forget that the M2 only has brightline frames for the 35, 50 and 90mm lenses. It is now on the reserve list, usually with a Visoflex attached!
Recently, I had thought of getting an MP a la carte, not because of any flare issue (to be honest I didn't know there was one until I read your post) but as a retirement present to myself. In the end I just kept with my M6 as it is an old friend of twenty years!
In all fairness, you really do need to go and have a play with one at a good shop: and buy it from there, not ebay!