The while cancel your warranty threat is illegal Harley nor any other company can take your warranty away. They have to prove the addition of the item caused a failure of an item to deny a claim. That being said I am sure they will fight claims of trailer owners and most will give in because of the money to fight the claim may exceed the repair.
I agree but like you said probably way to expensive to fight. Thankfully I've not had to face a denial of claim so I'm unsure is the moco actually voids the entire warranty or just rejects the specific claim on the grounds of having a trailer hitch (or other non-factory authorized mod).
And the ESP does not have any protections since it is a service contract not a manufacturer's warranty and is not covered by the Federal law. I believe the can and will void the entire contract (and give you a refund on the unused portion).
I'm just less willing to take the chance and have to be concerned should I have a warranty claim while on the road with a trailer. For the very high cost of these new bikes I want the moco to take care of any and all problems during the warranty period. The cost of the bike itself is more than enough 'Harley tax' for me, lol.