Hey mate, welcome to the site!
I'm no expert on the area, and no doubt you'll get plenty of info more relevant from the scores of blokes on here that practically live up there, but here's my 2 bobs worth:
Yes, it's magic country!! I've visited and or stayed in both Lady Musgrave and Fitzroy lagoons several times, and (in good weather) they are absolute paradise. Musgrave is particularly beautiful, with its beaches and turquoise waters. The beaches are actually coarse shell grit and dead coral, do watch it, if you plan to go ashore (strongly recommended).
The entrances to both lagoons are no problem, but it us best to use your plotter to make a safe route, during the day, if you plan to come and go at night.
Your rig would be pretty economical I'd imagine, but take heaps of fuel if you're planning a few days out.
I've only ever launched from 1770, but can see your point of Gladstone being a bit closer, particularly to the northern end of the group. Personally, I just love Agnes and 1770, and leaving from there and supporting the local businesses, is all part of the experience.
I'm up there next week with the family and can't wait!! Good luck with your trip, don't forget to let us know how you go!