Hopefully uncle Derek would give you a lift... like you say they can get pretty heavy after carrying them for short distances, your poor arms would be stretched & aching before you got to the end of the road lol!
thats a good idea emailing the vets for information if you can't find a recommended one on here.
experienced spaying vets would reduce the risk considerably for Pretty Fat

Extra bedding to keep her cosy is great too
i know you're concerned about the risks of having her spayed, but if the vet agrees with spaying at the moment, then whilst she is reasonably healthy she will be able to recover more quickly & be more resilient to problems.
the older she is & the more advanced the cyst gets the higher the risks will be. When she gets to the really poorly stage, the odds will really be stacked against her recovery
but yes if she has a URI then maybe wait a little longer until she is more healthy & the breathing is sorted before deciding on an op.
Just be aware that if left too long & the cyst does burst it will then be an emergency, very painful , very stressful for you and her & the chances of survival will not be good.
According to the amount of posts on here regarding ovarian cysts- yes they do seem to be fairly common, especially in middle to older aged guineas, but the information suggests that they are treatable & the guinea can lead a relatively long & normal life after treatment.
I had my Ellie treated for hers with the HCG hormone injections because she was 6yrs old & not considered for a spay due to the age risks.
Ellie died around 2yrs later of multiple health issues.
She seemed more relaxed & contented after the treatment, her general condition improved considerably & she seemed so much happier - the pain & effect of the cyst had been draining her energy levels, making her quiet & subdued compared to usual.
Ovarian cysts in humans are regarded as most painful when they are small or growing, however if Pretty Fat is 'bunny hopping' this is a sign of pain, i guess the same as having a stone in your shoe, something is the wrong place & making you limp because it is painful.
Do you hobble because of pain? this would be similar to her bunny hopping as it is not a normal movement.
Pain in guineas often causes them to not eat as much & then all the complications with digestion & teeth issues could be caused

I'm sure you wouldn't like to think that she was in pain? please sort her out some pain relief if nothing else.
If you could get her scanned/xrayed at least you would know more about what your dealing with &it could be confirmed if its definitely a large cyst.
i shouldn't worry about Simon not being paid for giving advice to your favourite vet, i think he's probably more interested in sorting out the best possible outcome for the guinea & passing on knowledge to other vets would possibly lessen his work load

i guess if he didnt want to share information, he just wouldn't.
see what results you get with your emails & get her treated by the one you feel is best, discuss Pretty Fats options with the vet & see what they can offer her - after they have advised you, it will be easier for you & nana Niki to decide whats best for her.
Good luck & let us know how she gets on.