We want to motivate our tenants to pay their rents ASAP so we charge daily late fees.

I want to make this work and am so motivated, but I also don’t want to continue to wonder if what I’m doing isn’t legal 100%.

But we have a lot of interaction with guests before they ever arrive - we require a photo ID of whomever has booked our house (and the name and info has to match the booking info), a working cell phone number that we will send a test message to by text to confirm it belongs to the guest (or is at least in their possession), and we converse with them to let them know what to expect from us (and if we have any specific concerns we share those as well) - for example, this week if we had anyone scheduled to arrive we would be discussing the incoming hurricane. 

And the customer service is super fast and helpful, although I should also mention you are often just sent a wall of text to read.

They wouldn't have to set expectations of everyone was doing it as they should.I have stuff set on auto.I get a text and usually answer in minutes so it's no big deal for me.

The neighbor is now sending me text messages about a tree that is hanging over the fence. 

Having skin in the game definitely makes borrowers more motivated to follow through.

. - The seller has no interest in keeping the property (I guess that makes her a motivated seller?)

Apparently the OP has been over whelmed with calls and texts from solicitors, marketers, etc. as a property owner.  

Give us some color… days on market, estate sale , divorce etc. what's the motivation