Thank you Lynda,
I can understand your distress, as this is a challenging situation. Without knowing whether this is a sinister or benign mast cell tumor, one cannot make a fully informed decision. And I have to say that this is quite common with this type of tumor since the small aspiration sample size can make interpreting these tricky cells difficult. Especially since mast cells are a normal part of the immune system and can normally be present anywhere there is infection or inflammation (which would go along with the current ulceration).
So, with this complication in diagnostics, your vet’s options are reasonable ones. Of course, it does depend on what they are referring to when they say “chemotherapy.” This is because this is not a tumor type that has really shown much response to chemo (plus it's less then ideal to use chemo without a diagnosis). Though if they meant palliation with steroids, then that would be a different situation (as it could reduce any swelling associated with the mass and reduce discomfort) and could be an option to consider as opposed to just leaving the mass to do what it will.
Now the first option you noted was anesthesia for further testing. Now before we even consider any anesthesia, his organ health is the most important consideration. Therefore, if you were to consider this option, it would be highly advisable to have a blood sample tested. This would let you confirm that his organs are healthy and that he is a good candidate for anesthesia. If anything was found (ie struggling kidneys, liver disease), then this option would be off the table for Mickey right off the bat. If it was clear, then it would be a viable option for him (and the murmur isn't a major issue if he doesn't have clinical heart disease signs, but would only mean the vet would potentially alter their anesthesia plan for him).
Otherwise, if he was confirmed to be an anesthesia candidate, then this option could be realistically considered. Though if this mass is small (at least as small as it will ever be), then realistically it would be better for them to rule out tumor spread and then just remove as much of the mass as possible. This is better then taking a biopsy (since that actually would have increased risk of tumor spread in the area). And as this location isn't one where clean margins are likely (since the cancerous concern we have is a tumor that snakes out tentacles into the tissues as opposed to just growing as a mass), the surgical approach would be the same for either.
Finally, there is the option of benign neglect. If this hasn't caused him bother until now, this is very tempting. Especially if the flare up was potentially traumatic as opposed to this growing in size. If it were, then one could hope there was scope for this to settle and resume its innocuous existence. The trouble though is that at this stage, we just cannot predict the masses future actions and if grows any bigger such that the skin cannot stay whole over it, then we could be stuck with a permanent festering wound. Therefore, this isn't an ideal option for Mickey if we can avoid it. As I noted before, it'd be better to use palliation (and treat any infection that may be present currently since it is ulcerated) then nothing at all. And if we did and the mass responded to steroids, then we could see him manage with this for a good wee while (likely months to years as opposed to weeks...but again without being able to confirm if this is cancerous and what grade we cannot truly give a prognosis.
Overall, it is not an easy decision for anyone. If you are considering surgery/anesthesia, then I would say that the first step is to check bloods. Because if he has any age old issues with his organs, that may not really be an option for him. If that is all good, then we'd be in a position of hedging our bets. We can remove it as it is to give him the best chance of having no further issue or at least prevent this from growing/festering as long as he is with us. Or if you don't feel that is the option for him, then we'd want to consider further discussion on your vet's chemo thoughts and the use of steroids to reduce tissue inflammation and at least try to keep this as small and comfortable as possible for as long as possible.
Now I will pause there since I did type quite a bit to you. Please let me know if there are any more questions you have or anything I have missed (as your history was quite verbose as well;)).