Any moisture intrusion should be disclosed, but I would ask.  

I'll be grumpy about the intrusion, but I am going to treat this loan as if one day they will come knocking... if/when that day comes, I won't have anything to hide. 

My question is: Can I just go knock on their door instead of sending the letters or is that considered intrusive?

These have been around a REALLY long time - the roadside "bandit" signs were used before technology allowed more intrusive techniques.

I also noticed a tree with roots that are butting up directly next to the foundation and siding of the house where I believe so far the water intrusion occurred.

But they are not bothersome, as they are not "intrusive" on day to day life (I look at the mail when convenient - it does not interrupt my day to day life).   

We feel it isn’t too intrusive and helps educate the guest on why 5-star reviews are helpful...

The more intrusive your method to reach them the more flack you will get in return.

It's whatever is stipulated in the contract.Generally one company does it only but who pays for it seller/buyer or both and by what percentage each covers if it is both is negotiable.The contract can also stipulate which title company is used.I hope your buyer is having an Phase 1 report,vapor intrusion,site inspection,and cost reserve stable completed on the property as part of the due diligence.All of those items run about 3,500 to 4,000 total and can be completed by the inspector.

Will definitely try this, as it will be less intrusive than a phone call, but probably more likely to be answered than direct mail/postcard.