Would a Retainer Fix my 3mm Gap in my Two Tront Teeth? if So, What Type Would I Need? (photo)

This is rather large bonding and after ten years it is completely normal to have some staining.  It looks as though your best option would be to have to porcelain veneers placed.  These will aesthetically look much better and will not stain like dental bonding.  Good luck. 

Tough to say exactly what to do without photos. If you expand and you have no crowding then you will end up with spaces. If you have crowding then expansion is an excellent way to help relieve it. 6mm is a significant amount of crowding but well within the range of treatable.

You are experiencing what's called parasthesia.  It can result from an extraction during which the nerve was damaged.  It can take years for the feeling to come back.  Speak to an oral surgeon.