It's not anymore. Fully independent since 2010, though I am sure they have financial support/defense arrangements in place. Own money, Guilder, own laws - no (legal) cannabis in St Maarten.
St Martin, French, is still really French, including Euros.
As to slice, well kinda awkward to talk about a half as there is no overall island name to St Martin/Maarten.
Unlike say Hispaniola for Haiti n Dominica's island.
actually, https://en.m.wikipedia.org/wiki/Saint_Martin even supports El Reg, though I've rarely heard St Martin in a context meant to encompass both. The Dutch, for one, wouldn't dream of doing so.
Flights? Charters, never flag carriers. Air France's always been a gouge there.
Island was divided in 1648 and avoided all Euro wars since. Wags have it the division was by country representatives heading of in opposite directions on the coast. Wherever they'd meet opposite would the border. Legend has it the French runners got the Dutch really drunk the night before, hence much bigger French side.