Why does this change anything? Even if there will be a new island Jinbei can't join the army. Nothing would change.
It will become even more worse if the king and princess have a wanted criminal up in their ranks. The WG will be truly pleased by that.
By that logic he could be still at the army now
Coming for under the radar visits from time to time and reentering service are pretty different things. Especially for a country that's attempting to join the reverie.
He chooses to remain outside of the country whenever possible for a reason, and that reason is not likely to change.
But he can't be in the army now, because FI would get unwanted attention. No FI is open game in my book and fishmen in the outside world, not just reverie, is something I don't think the world government would want. Especially them talking about slavery, some malcontents hunting humans and a very dangerous future queen with the power to summon Sea Kings. Something all governments share is a hatred towards change, especially ones that can upset the balance of the world, and Fishmen on the surface isn't something they'll just say "Oh well what can we do?" They will fight this.
Shirahoshi will need all the help she can get, allying herself with an infamous Fishman pirate may just be the best thing for the people in the long run. Shoot even if she doesn't ally herself with Jinbei, this won't stop WG from thinking they're allied anyways. I mean he could always just be a secret advisor as well.
Although this is all speculation. I think we're gonna see Big Mam at some point(who's of course Lola's mother) and that excerpt from the last page about Hacchan saying "there are people much stronger than Hodi"(or Hordy) around makes me wonder if he really is the main boss.