Yep, you don't want to put any load on the spoiler if you can help it. High mounts are better for this as the top straps can be higher, or horizontally aligned with the top of the spoiler. You can get low mount carriers to achieve this too in some cases, but you need to know what your doing ;0)
Depends on usage really, if your looking to transport bikes more than 5 times a year - I'd recommend a roof mount option every time ;0)
As for covering the number plate.... Most carriers (or the bikes on them) will obscure the plate in some way, yes it's illeagal, but you have to be really unlucky to get a ticket for it