I know it’s nitpicking, but technically bananas are not trees, they just somewhat resemble trees. It’s a banana plant.
I think you could successfully grow it at an angle, but you’d need to support the weight. I assume your greenhouse is heated to prevent a freeze?
I would say take any claims from sellers with many grains of salt. My understanding is basjoo (which is almost certainly what you have) will not fruit at all unless pollinated, and when it is pollinated it has tiny fruit that are almost entirely filled with large gravel-like seeds. The small amount of edible flesh around the seeds has a very mild flavor and isn’t particularly sweet.
My own attempts to grow seeded/hardy bananas here in Seattle have mostly failed, though I do still get one Helen’s hybrid that dies back at first frost and then grows again each summer, but it’s never going to fruit. There’s some info about seeded bananas linked from this thread: