Have you found more expensive insurance is simply "the nature of the beast", or is there a better way my broker and I can go about keeping costs down on that side?

Ive seen many attempt to DIY on their own form a distance and fail.  

I'd encourage you personally to either consider a strategy that allows you to add value quickly and get out quickly (a Flip or BRRRR) if the HELOC is your current only/best option for cash, and/or to simply save up.

I simply call my broker and add the customers name to my liability policy.

If you do not close on enough of your properties to defer all tax you will simply pay the same tax you would have at the same time you would have.

This is the first time I'm attempting this strategy, so I'm kind of flying blind. 

Don't give names or addresses, but simply ask hypothetically. 

Seems to me whatever % you would give up would be offset by simply renting out the upstairs unit.    

In your case, I would simply point out that they forgot to give a value for that space.

At some point you have to accept that your first attempt may not be perfect, but if you set some guidelines and narrow your focus down to something that's achievable you should be on your way pretty quickly.