welcome to Health care magic.
1.It looks like sailoadenitis (lump from the
salivary gland infection / obstruction), since it been there for a while, gradually increasing in size.
2.I don't think there is any relationship with the mucus collection and lump.
3.You said FNAC - inconclusive ( most likely its cystic).
4.If you are my patient i would have examined and ask for a
ultrasound neck ( helps to detect the nature of the lesion, its origin and extensions.
5.Only if its inconclusive an CT would help in further evaluation.
6.So follow your GP as recommended.
Hope it helps you. Wish you a good health.
Anything to ask ? do not hesitate. Thank you.