I'm in the much the same situation, allthough I think things are sorted now, I hope!
So how qualified to get paid work? It seems like the shores are crawling with people looking to get a paid position as crew on a yacht, delivery jobs etc. The problem people have is finding the skipper. A skipper needs an ICC (International Certificate of Competence) in almost all cases nowdays, for insurance if nothing else. In other words, experiance and an RYA quali. like Yachtmaster offshore or Ocean. Paid work without experiance, I'd say small chance.
Unpaid, or "pay your way/shared expenses" sailing is the thing to go for... and even that people would like to know that you'd be of some use and not hanging over the side being sick for the entire voyage =) Don't loose your hopes though, I think it's very possible for you to get a place on a yacht even though your completely un-experianced, plenty of people do. You have the fact that your a girl going for you... and I don't think that is just male envy speaking. 6 months is ample time if you've got the money... However I don't think it is enough to get paid work guaranteed, simply bacause you wont have time to put enough sea miles under your belt.
I dont know much about your second question... Allthough I've heard that hanging around a marina that is the start-point for a lengthy voyage like Antigua for a transat is not allways so successfull. People want to get to know you through short day cruises etc. before they take you aboard for a long journey across an ocean.
'Female only' crew jobs? I don't know because I'm a guy. However they do seem a bit 'dodgy' like you say. There are of course All Girl boats cruising. I just wish they'd let me join them, as a small exception to the rule
Anyway, good luck!