Hi and welcome
I am very sorry.
Please see a vet again for ovarian cysts and give them a call again tomorrow morning when the clinic reopens. How experienced are they with guinea pigs?
Crusty nipples and thinning hair on both sides of the belly are typical symptoms for ovarian cysts. The hormonal ones that cause the classic symptoms tend to be most active between the age of 2-4 years; they are usually smaller but growing cysts. By far not all ovarian cysts cause visible symptoms and not all cysts are hormonal; the large fluid filled ones become more common in older age. Hard ovarian cysts (thankfully a small minority) can pose a cancer risk. There are various ways on how to treat ovarian cysts that cause problems; unfortunately none of it is cheap.
Here is more information:
Sows: Behaviour and female health problems (including ovarian cysts)
Recommended Guinea Pig Vets
Please also switch from the regular once weekly health monitoring weigh-in on your kitchen scales to weighing daily first thing in the morning when the daily weight swing is at its lowest for best day to day comparison. If you have never done that, please feel around the ribcage whether your sow is still a good weight for herself or whether she is underweight and needs medical treatment. Ovarian cysts often come with a noticeable weight loss. Just feeding more calorific food on spec is not a longer term solution; you will need to treat any medical issue for the problem to go away. Cheap scales from a supermarket or online will do for this purpose.
Weight - Monitoring and Management
How To Pick Up And Weigh Your Guinea Pigs Safely (videos)
The hair loss doesn't sound like either mange or hay mites to me.
New piggy problems: URI - ringworm - skin parasites
All the best.