2- Have only briefly driven an E70 with RFTs...our E53 has reg tires. Fwiw, I went with non-RFTs on my '02 VetteVert, and the diff was like night and day. Do some searching using the Search Tab, upper right hand corner on RFTs vs non-RFTs.
Additionally, surprised the fronts need replacing, and replacing at so few miles. Are the rears a 2nd set? If rears are orig, still surprised the fronts only went 13-15k miles. Something may be amiss, or they ran them with 20 psi and carved corners...
Not sure how non-RFTs would 'feel' with RFTs in rear; you may get posters warning about the 'tire difference' vis a vis the Xdrive system. Hell, you may get that excuse from a dlr when you need warranty coverage.
3-I GOOG'd Hendrick's 'warranty' but none of the links offered details other than bullets on selling a complete 'pkg' of warranty, maintenance, tire insurance and detail work. There were also zip 'reviews' of that warranty; not saying it is good or not good for you, but why worry about it 'today' when you have considerable time and miles left on your mfg'er oe warranty?
Nice car, welcome to a very good board, and I used to do some good biz at the the GM Tech Ctr, Milford MI, back in the early '70s.
Saw some amazing test cars, prototypes and mules...
GL, md