I'm trying to imagine the extra keys you speak of, for G/F? I don't know really good advice for prevention of clogging - I loathe anticondens, it tastes disgusting when you end up having to suck back (because it doesn't prevent the need for it totally) - but you produce most saliva (apart from during a meal itself) after eating, and after doing teeth, so best to avoid those two activities directly before practice. Weak notes can be a matter of the instrument's internal gremlins, but it's worth checking that the pads of the keys you have are sealing properly. Certainly when buying a new instrument, I do go through its range looking for duff notes and poor tone: when I bought my bass, I'd been intending to buy one in cherrywood, but on the day, the rack of cherry instruments all had weak low As, as did all but the one pearwood one I ended up getting instead.
Another problem I do have with weak notes relates to not checking properly that I've really got the various sections together for the best ergonomic fit for my hands on assembling instruments - and that happens to me embarrassingly often
As far as alternative fingerings go, difficult to advise - it's more a question of fiddling around and seeing what you find to work best yourself, I'm afraid, but a good starting point would be one of the extended alternative fingering charts. I know that one of Walter van Hauwe's books has them in, but can't at the moment remember which volume....