yes they will, europe is a single market, they cant not honour the warranty
Seems you know something the rest of the bike trade doesn’t! There are all sorts of get out clauses that can be invoked for grey market imports. The buyer still has a contract with the original retailer though, so any issues and you’d post the fork back to Germany, rather than to Fishers.
As for a 26″ wheel not fitting in a 650b fork… Think about it for a second… A 26″ wheel is an inch smaller in diameter, which means it has a radius of half an inch less. Meaning that if you fit a 26″ wheel in a 650b fork you will gain half an inch of mud clearance. Downsides? For the same amount of travel, the fork will be 10-12mm taller, so if you were going to run a 160mm 26″ fork, you’d run a 150mm 650b fork (still with a 26″ wheel in it) to keep the same geometry.