Not safely, no. Get a proper barbell.
Well it looks like a navel piercing with a navel gem barbell or a captive bead ring in it.
if its new not alot.if its an older piercing it does not matter
You can keep your navel piercing until you start to show, then see your local piercer and get a flexable barbell put into the piercing. It's call a PTFE barbell and it will allow for the piercing to stretch without tearing the piercing. However be sure you are getting a PTFE barbell and not some other material. PTFE is biocompatable and will be accepted by the body without issues, I have used these for hundreds of my clients without problems.
As you would without a navel piercing, just be sure the navel piercing is the last thing you clean before you exit the bath.
Lose some weight? Buy a smaller Barbell?
No a navel banana bell is not designed nor intended to be used in a tongue piercing. Tongue piercings are done with a straight barbell, the curve in the navel bell will cause trouble with the tongue piercing as well as being too short.
Years ago that was your only option, however now you can have the piercing done with a jewelled navel banana bell (basicly a bent barbell with a facetted stone in the navel ball).
Yes you can get it repierced however I would suggest a bent barbell or a navel gem this will avoid external irritation and allow the piercing to be done a bit deeper than the first time around.
No. Try to move it as little as possible. When you turn and move the barbell, germs and whatnot from the exposed barbell are pushed into the piercing and that can cause infection. Of course, some movement is inevitable during cleaning and just from you moving, but the point is, dont move it yourself :)
Your question "Can navel piercing migration be down to growth" makes no sense.
Well I would suggest taking a bath and soaking the piercing for a while that will loosen the blood build up on the barbell. Then before exiting the tub use the shower to rinse the piercing and your navel out with warm running water. Cleaning is a daily thing that can be forgotten when it comes to navel piercings.