Thank you for the kind words, he is a strong boy, so whatever it is, I believe he will make it.
Definitely demand that Princess gets a proper check-up, where I was I was told they were guinea pig specialists, but after finding out they only learned to grind their teeth and nothing else I went elsewhere to make an appointment.
I also recommend finding someone who really knows the guinea pigs well and will give you a proper diagnosis.
At the other vet they told me that by taking fluid from the lump they would find out if it was an abscess, if there were inflammatory cells which unfortunately could be cancerous or if it was a benign tumour. If I remember what the vet told me correctly, there's a good chance it's cancer if the moment you palpate the lump it sort of holds tight and you can't quite feel the shape of the lump from the back. But don't take that information 100% I'm not sure I'm remembering it right. That momentary lull is very nerve-wracking, especially when they're not at their best. My little guy has a lump further down his neck, but if Princess has it closer to the surface of her skin, maybe they'll offer to give it a try without putting her to sleep for a while.
Definitely, if it's in your power, I would recommend syringe feeding every two hours at least 15-20g of critical care, it worked for me when he was losing weight, it only stopped the weight loss at first but then slowly he started gaining weight too, for me it was worth cancelling two holidays and part-time jobs.