Well, shirahoshi with her ability should be plenty capable of defending the island from the yonko. Of course, there are risks in that too. The world government is sure to react to sea kings being controlled and revealing her ability to the world would bring plenty trouble to say the least. Also, the defense would work under the hypothetical scenario that not one strong person reaches the island and kinda the same applies to the soldiers. A single actual strong person from big mom's crew would ultimately decimate the island if he reaches it considering that the 70k soldiers plus the army plus the princes are not a match for people with real power.
No i don't think the Sun Pirates will join Luffy . Only Jimbei will join . The others will remain to protect FI IMO .
As for the thing with the pirate flag . FI has the flag of the SH's showing to protect them as far as i remember . King Neptune himself said that.
As for Luffy not keeping his promise i seriously doubt that . When has that even happened ? He always kept his promises and tried reaching his goal even if at some point it cost him 10 y of his life . Luffy is the type that keeps his promises and he promised to make FI his turf so we should get accustomed to the fact that FI is gonna be his turf no matter what .
I kind've think that is one of the issues he needs to handle, cutting ties with big mam. And telling his crew mates he is joining a human's crew, some will come some may not. Sun pirates don't need to protect FI they got 70k new recruits on top of what they already have. I think altogether this is the beginning of a larger force which will gather under luffy's banner. I hope the sun pirates come with Jimbei, maybe even the return of arlong.