1. Yes, you can port your old Sprint number to T-Mobile prepaid, as long as your Sprint account is in good standing (e.g. you don't owe them money). The process can take up to a week to complete though. YMMV. Your Sprint account gets cancelled when this happens, too, just FYI.
2. My thinking is that you SHOULD be able to still forward that other number to your new handset through Google Voice. Just keep in mind though that T-Mo prepaid doesn't offer call forwarding as a feature on the actual handset number, so if you intend on using GV voicemail, you have to go "all-in" with Google Voice (i.e. only give out your GV number and not the handset number). In this scenario, it's also a good idea to call T-Mo customer service to get your carrier VM turned off. This will prevent missed calls to the handset number from being picked up by carrier VM.