Couple of things.
I have a 1978 R100RS with snowflake wheels and I know that the discs will interchange as over the years I have swapped discs around to keep (for no other reasons than asthetics) two BMW OEM discs on the front of wife's R65. I very much doubt that you have made an error in measurement so I suspect that the differences in fork spacing may have been taken up by the cast on bosses at the bottom of the early forks. In any event, for a brief period I flirted with the notion of fitting spoke wheels to the RS. I still have the front somewhere which I recall fitted perfectly, unfortunately the rear was a drum model and I decided against converting back from disc to drum.
Lastly, I sympathize with you as a fellow sufferer of the dreaded ATE "swinging" calipers. They must surely represent the only instance where a motorcycle manufacturer went from drum brake to single disc and then almost immediately to twin disc because the single disc was less effective than the drum brake it replaced.
I never got around to fitting a late model Brembo front end to mine, and of course now the antiquity of such beasts means it will stay "stock" in the interests of originality.
As far as I know all single disc R65s up to the end of the twin shock model have caliper mounts on both fork legs. I am willing to be corrected on this and would like to see a photo of a pre 1985 fork leg w/o mounts.
Admittedly it was a long time ago, but when we bought the 2nd disc kit from BMW for wife's R65 (twin ATE Brembo copies no less) I do not recall the kit including a new axle or spacers, which stands to reason in that BMW would have had to have a very good reason to mount the front wheel anywhere but dead centre between the forks.
As regards the master cylinder, the cost effective way to fix that is to buy a rebuild kit in the correct size (it's 13mm for twin ATE R65s, not sure what size for Brembos) and then have a good brake shop bore out the existing master cylinder and press in a stainless steel sleeve. Not only will this work perfectly, the stainless steel sleeve means that corroded master cylinders are a thing of the past and any subsequent rebuilds require only a new piston and seal kit (or just a seal kit if you can find them). Last master cylinder I had bored and sleeved here was $100 and a piston kit is about $40 from Motobins.
A suitable brake splitter can be found in VW type IIIs, it even has the right sized mounting hole. It varies from the BMW part in that it is made of steel not brass and has 4 holes instead of 3 - a perfect invitation to also fit a VW brake light switch as they are more reliable than handle bar switches.
