How long does PMMA last? Worth excising small granulomas?

I'm sorry to hear about your experience with "lumps" [We cannot call them granuloma unless they are cut out and we have a pathological diagnosis of granuloma. Just know that all "lumps and bumps, are not necessarily granulomas. 
I have used Artefill [now called Bellafill] fairly extensively for the past 6 years. Because the product has only been on the Market in this form for the past 7 years, we can only say it lasts "7-years", truth is, the PMMA [implant] becomes surrounded by your own collagen, so it does last forever. Over time, we continue to age and loose collagen, and your lips have become thinner, hence while you are noticing this now. The PMMA implant cannot compensate for ongoing collagen loss, which is why I still encourage my Bellafill patients to come in annual to see how they are aging. 
As for Bellafill in the lips, It's really not a good filler for this area of the face [nor is the under-eye area] because in both locations, there are round muscles that go around the entire lip and eye respectively.  These muscles tend to "pull on the implant" and move it while your body is forming collagen around the PMMA beads and so there is a risk of developing small round lumps as a result.  In your case, these probably developed when you had the lips treated, but you had more collagen in the lips overall, so it may not have been apparent until now. 
Placing Juvederm around the filler is one option, the other is to see a provider who is experienced with Bellafill as they may be able to inject the lumps and remove [or at least shrink] the lumps first. And YES, Bellafill can be removed with certain medications, it takes time and expertise, but it can be done. 
An additional piece of information for your consideration, please be aware that all fillers can cause lumps and bumps. In the U.S. pivotal trial done in and around 2009, the reported onset of a granuloma 3 months or more after treatment was seen with less than 0.3% of treated patients. This rate is similar to all other fillers. So, while placing Juvederm in the lips to restore the missing volume is a great solution, know that you may still form additional lumps and bumps in the lips.  
Final take home - don't have surgery, you will end up with even less lip volume and have additional scars to content with.  I doubt this will improve your satisfaction rate. 
Hope this helps. 

You should have never received artefill in your lips because it simply does not belong there or for that reason in many other places on the face and body. Microbeads of pmma cat last nearly a lifetime of a patient. Visit a plastic surgeon who can evaluate you, make his recommendations and treat you as appropriate. Good luck from New York City