I've never seen this, then again I don't tariff the crap out of my CNs and generally don't get the Caribbean because it's all annoying islands to deal with (I also generally play rnw), let the ai have that headache, I'll take 'murica...
However, there is a purpose for the ai in building those forts, you just don't agree with it. If the province is connected to a fort it has to be captured for the land to be taken so the ai tries to make that happen for most/all their provinces. See this forum for the rage having to siege all those island forts to take them causes some players...
No.
Caribbean has never fought anybody at all, and this will be 2nd time I'm paying off debt there. Peru declared bankruptcy, despite that I paid off its loans twice and that it had 0 OE with 1/2 true faith, 1/2 pagan when it came into existence.
La plata accrued debt without actual fighting as well, same with Louisiana. Mexico at least saw a little fighting and conversion, but have also required > 1500 ducats of debt payments. In all, before 1600 colonial nations that have fought no old world AI have accrued > 5000 in debt, including thousands on a couple that never even fought natives.
The only other new world than mine belong to France (my ally), in Canada. These nations all have sub-50% tariffs, most have sub-25% since I don't spend monarch points to raise them at all this run.
The AI putting up thousands of debt between them in this scenario is inane and inexcusable. It's abject game throwing and no amount of playing pretend WRT forts "protecting" 2 islands or even just 1 (it later built a fort on one of the non-strait province islands) can justify Caribbean being > 300 ducats in debt with no fighting, conversions, or overextension in its entire history.
Game throwing to be annoying is not sound play. It isn't even reasonable play.