Hi KCM,
Since you work at a vet practice, i think an easy and minimally invasive procedure to perform is a fine needle aspirate of the lesions. I personally perform plenty of fine needle aspirates. It can provide a lot of information quickly. Punch biopsy is a bit more invasive so i might hold back on that.
Imaging can also be useful. You can also consider ultrasound using a small sector probe. If gas bubble is a cause, the air silhouette will show on the ultrasound (Air looks white/opaque on Ultrasound). I am not sure whether radiography will be of any use practicality wise since the lesions are in an awkward positioning but can also provide some information.
Cytology and microbiology can then be done from the aspirates which can give an indication of the underlying pathology. You might like to culture some of the aspirate especially for coliforms, blood agar and gram negatives.
I personally will not use Furan-2 until i have at least collected a sample for diagnostic testing. This is because you want to be able to detect presence of any infectious aetiological agent. As a general rule, do not use any antimicrobials/antibiotics until a sample is collected. The reason being that preemptive use can complicate the intepretation of results from the testing (false negatives).
Secondly, without knowing the aetiological agent, use of furan-2 can also result in 1) antibiotic resistant strains developing. The objective in the use of antibiotics is to completely eradicate (bactericidal) the bacteria or halt its proliferation for the immunity to rid it (bacteriostatic). Using a 'weak' non specific treatment like furan-2 risks resistant strains getting a foothold. In addition, the lesions appear walled (abscess/cysts/bubble). Nitrofurazone and furazolidone, the active components in furan-2 are not usually able to penetrate deeply pass the dermal layer into walled lesions as a bath. If antibiotics are indicated, it is more effective via an intraperitoneal injection using amikacin and piperacillin.
Thirdly, using furan-2 can actually promote a secondary resistant infectious agent to thrive. For example, it can help with gram positive and negative bacteria but not against some fungus and parasites. Normally in mixed infections, there is competition among the various microbial agents. By selectively wiping out one group of them, allows the other group to thrive better. This again complicates intepretation and treatment.
Cheers.