i agree with the others in that try treating her for lice/mites is firstly, if she is stressed she may be susceptible to them & the others may not get them at all if they are younger, healthy & stress free - but looks like you have already treated that without success
has her hair loss got a symmetrical appearance? As in both sides thinning hair as well as her rump area going bald?
When you say she's 'independent' do you mean as in a hormonal woman type attitude? i'm just wondering if ovarian cysts could be a possibility?
My 6yr old Ellie had ovarian cysts discovered by a conscious ultrasound, she didn't have any of the text book symptoms of hormonal grumpyness, symmetrical hair loss, but she did have recurring UTI's &terrible bleeding when she pooped too. she had the HCG Chorulon injections which sorted her in a very short space of time - her coat went very soft & silky 24hrs after the injection & not coarse as it had been before. Although i hadn't noticed her be particularly grumpy, she did mellow & chill afterwards too

she had one bad bleed afterwards & never bled again, the cyst grew back 12months later, so she had another 2 injections & died of multiple issues at the age of 8yrs.
growing cysts are more painful than established ones, so if its painful she may be pulling her hair out as a pain reaction? can you discuss with the vet trialling some painkilling medications? maybe Metacam/loxicom - although if she's drinking lots she may have some kidney issues going on? see if your vet can give you any advice on that too?
i would only allow steroids as a very last resort on any of my guineas, trying all other options first.
Healing vibes for your pretty little girly

Speedy & full recovery, luv the girl & I.xx