Toyota isn't going to spend millions or more developing, testing, and certifying an engine for 1 vehicle.
It would have been cheaper. The warranty costs are going to eat them up. I don't know anyone with a BMW that doesn't have a great relationship with the BMW service shop and loaner car department. They always offload the thing before the warranty expires too.
No, it really wouldn't have. Molds to cast engine blocks alone can easily reach a million. Then you have machining time on top of that, which isn't cheap, considering they likely have no way or room on their current lines to accommodate a straight 6.
Then once you finally have a working prototype, you have to test and tweak it, again time intensive and not cheap. Only then can you certify and test for emissions compliance. For a low production sports car, it makes absolutely no sense spending all that money to develop an engine when someone else already has.
The B58 engine has only been out for 4 years, and is a massively different than the previous trouble prone N54/N55 engines. You can't fairly compare them on a reliability standpoint.