My property manager has a tenant who is consulting a lawyer over a dispute regarding pets.

If the affiliation isn't very strong, conflict resolution becomes a problem.  

What do you think is a realistic outcome/resolution?

When I upgraded my computer, the screen resolution changed to 4k, and since QuickBooks Desktop version doesn't support 4k, I had to find an alternative.

Is it the lien holders responsibility to dispute the claim and prove that they weren’t notified or is it my responsibility to prove they were? 

I do like your buy & hold strategy, but unless you really enjoy driving to tenants to collect rent every week, you should consider requiring your tenants to sign up for automatic withdrawal from a checking or savings account.Collecting rents in cash creates the opportunity for payment disputes, not to mention being a security risk and a huge inconvenience for you.If you're goal is to build a serious portfolio of properties, you'll need to resolve the rent payment issue sooner or later.

One of my new Year's Resolutions was to be more active with them.  

I don't think domestic disputes can qualify for "illegal activity" From the info given I don't think you have legal grounds to evict at this point.

Disputes seem to come up with expected use and when someone wants to sell.For example - brothers A, B, and C went in on the investment but Brother D and E didn't. 

Provisions for dealing with disputes (arbitration or mediation, etc).