Is commissioning into the Navy as prior service Army achievable? Has anyone had any experience with commissioning in their 30s?

Doable? Maybe, but the fields are limited. Some cap input prior to 29 years (SWO). Others vary. You can look up possibilities on the Navy officer recruiting site and look at the eligibility requirements and read up on the varied application requirements. All that is skating the short program. The long program is a different story. Never, never, never get in a holding pattern with a generic enlisted recruiter. Your package needs to get in front of an officer accessions type aka a real officer. If you pass the initial filters (50+% won't), then your package goes in front of a reviewing officer who typically does a deep dive interview and makes a judgement if you're worth the investment (60-70% of the prior 50+% won't). You are competing against others. Your degree in criminal justice equals "generic" as you need a LLD etc. for JAG. Many designators have "preferred" degrees so you're already behind others there. Reserve programs have essentially the same process. I was a long term Reviewing Officer, hence was involved heavily involved but obviously dated at this point. And, like everything else, the number of slots in any particular designator varies every year, hence different pucker factors with the competition. BTW, your ENL record will be looked at early and often. Better not be any boogers there.

In Navy OCS, just like Army OCS, you don't get to select your branch after you graduate. You could just as easily be selected to transportation or quartermaster branch as you could for MI.

You could join a Reserve or ARNG unit as an MI officer, the reserve component is manned by filling slots so you find a slot that needs MI and join. Don't expect to join the reserves and come active, it's much more complicated as an officer.

You should contact you local Nary Recruitment Division for guidance.