MJ, sorry to hear that you have problems.

Reading through your description, swelling is not normally associated with MG. Pain is not caused by MG (according to all the MG Neuros) though it can be a secondary effect of fatigued muscles.

On the whole it sounds like MG. Being sero negative, will make getting a DX more difficult. But a Mestinon trial is IMO a very good idea. Mestinon only stays effective in the body of 4 hours, and it starts working after 30-60 minutes, so it should be clear if it is helping and it is easy to stop if it is not.

Ask the Neuro or GP if you can trial it and let us know how you get on.