You have great questions.
My recommendation is fine needle aspiration (FNA). In general if surgery is to be performed on a STS (soft tissue sarcoma) it is preferable to know BEFORE surgery. Therefore FNA is a good choice. Hopefully a benign inflammation will be found.
How long can granulomas hang around?
It can be a long time. Some granulomas turn into scar tissue so it is hard to give it a time line. Warm compressing can help resolve granulomas. The warm compressed increase blood flow to the area.
Risks of metastasis following surgery...
These risks also follow FNA. Unfortunately there is always risks. Why is there risk of metastasis? Any inflammation of cancer cells can cause them to metastasize. This is why FNA is a good choice - less inflammation and if a STS is present then very AGGRESSIVE margins can be made in the hope that no cancer cells are left in the wake of post surgical inflammation. This is also an advantage of lasers employed in surgery - they can ablate and destroy the cells at the incision site while at the same time minimize both pain and inflammation.
Note: benign masses can NOT metastasize. Metastasis is a characteristic of the malignant.
Hope that helps.