We use a 3rd party company that specializes in only property management trust accounting (nothing else), and they reconcile the bank account every single day.

By negotiating effectively, we reached a deal that suited both parties, allowing us to focus on planned renovations that would significantly increase the property’s resale value.

They let my on-site manager know they were going to "take a trip for a birthday party" and have not returned. 

Second position lienholder have done some shady stuff in order to delay a loss, including filing suit against all parties or trying to obtain a temporary restraining order.

However, you would have to rework your current contract to be subject to SS approval  (which the buyer may or may not be into),or sell to another party, and if you are getting financing for the new house, that will likely fall apart as well. 

If there is no Operating or partnership agreement tell both parties to never invest in this way again.

Is it worth having tenants use a 3rd party service/app for collecting rent, doing maintenance requests, etc?

You want to make both parties happy.Most wholesalers fail because they are chasing the spread (their money) and not focused on helping the seller and providing a win-win for the buyer.

I have learned from property management that boundaries/expectations are a must for both parties so that we can come to the common goal and avoid the burnout.

One to diagnose and one to fix after a part is ordered.