Just starting journey…

welcome to the forum. Does anyone hold power of attorney for her? Your husband likely needs to have it for both parents, as is seems unlikely that your FIL can continue as the primary caregiver. As it seens likely that she is going to resist care, you may have to pursue guardianship. You need a certified elder law attorney, look for lists by location at nelf.org. Sometimes an experienced attorney may be able to convince her to sign power of attorney papers (for healthcare and finances) in the context of everyone in the family doing it--ie if her husband signs too, she might consent to do so. guardianship is more expensive but may be necessary anbd unfortunately you probably have to take the legal steps first before you can force the medical ones.

Hi and welcome. I am sorry for your reason to be here but pleased you found us.

This sounds like a difficult situation all around. It sounds as if you live at a distance. Some thoughts-

1. Your husband needs to get the legal pieces together asap. His dad needs someone other than his wife as his legal representative and it's possible dad isn't up to making decisions on behalf of the wife at this point either. It's hard to say whether an attorney would deem her competent to understand what's she signing vis a vis a POA; your DH might need to get guardianship.
2. Dad might have had a cognitive shift as well. Sometimes both people in a marriage have dementia and one flies under the radar because they're not yet as impaired or are just generally more agreeable. Those toenails speak volumes about his competency in managing her care-- APS would deem her "neglected" given her diet and toenails.
3. It is probably prudent to move them nearer you where you can keep a closer eye on their care. If there's unlimited funds, you might be able to hire a Senior Care Consultant and bring caregivers into the home but that's expensive and hard to manage at a distance.
4. Both probably need to be seen by a PCP. MIL needs to start the diagnostic process to rule out conditions that mimic dementia like a vitamin or hormone deficiency especially given her diet. You mention an exam for insurance purposes-- do they have LTC insurance? If there is a POA, does your DH need a diagnosis or doctor's note to activate it?
5. Ugh, the toenails. One of my aunts had this issue. The pictures will haunt me forever. She wouldn't let her live-in DD address this and things got really bad. A PCP can refer to a podiatrist who might need to schedule the procedure in a free-standing or hospital surgical site.
6. Given her combativeness, she may need a stay in a geri-psych unit for medication before you can do anything else. If she acts out aggressively, you can call EMS and have her transported to the ER. It might be useful to know which hospital near you have a geripsych unit-- not all do. Best if she could be transported there. Good luck. This is some challenging stuff.