When I had an active business, it's in an LLC, I bought liability insurance for it, and my insurance agent advise I should buy an additional endorsement that covers me personally as well, as people sue the LLC and it's owner, both, by default, and she's seen it many times.

There are a few companies that can endorse the homeowners policy to reflect the rental and add the charge for it but that is rare.  

Opinions expressed in this article are the author’s alone, not those of a third-party entity, and have not been reviewed, approved, or otherwise endorsed.

You can operate under your name or form a separate LLC/CORP strictly for the STR (do an S corp Election ie: consult with your CPA/attorney) See which avenue suits your need and please get STR insurance endorsements.

My attorneys as well as my insurance agent insist I buy an endorsement with my LLC policy that covers me personally as well since litigant's sue the LLC's and owner by default, normally for negligence. 

If the estimator your company is using is updated, then you would probably need to have them rerun it next years renewal and have the new value endorsed.

Benjamin,The Umbrella policy generally just provides additional limit of Liability if the underlying limit of the Homeowners, Dwelling Fire, Auto, boat, etc. is exhausted in a claim.Most of our carriers that offer Water & Sewer line coverage offer it as an endorsement on the Homeowners or Dwelling Fire (for 1-4 family rental dwellings) policy. 

I recommend adding "Personal Injury Liability" endorsement to the policy (protects against Liable, Slander, Wrongful entry, ...). 

You must have broker license with endorsement in management, or you can contract with a single owner to manage their property.

I recently received an email from bigger pockets and a turnkey company was sponsored in it, does that mean it is reputable/is that an endorsement from Bigger Pockets?