I can see your problem getting the right chainrings and chainline. Can't think of any solution right off the bat.

Hope your wife enjoys the bike. 750w is a lot of power on a bike. If the cables are getting too hot then they are simply too thin!

As a very fit cyclist (not pro or even close), I can put down 175w for 4 hours and that's working hard for 4 hours. 300w would be great! I could sip a mai tai and enjoy the scenery more!