Gotta know your capabilities and gotta do the math on the worst case.

If you have a good attorney it would be smart to show your husband the actual math of the scenario.

Don't use napkin math 'rules' to make investing decisions on.I hate the 1% rule, for the same reasons I hated the 2% rule a handful of years ago. 

It's a math problem that you can easily figure out  

A HELOC carries a higher rate over a shorter amortization usually 10 or 15 years and a mortgage carries a lower rate over a 30 year amortization offering a lower payment.You have to run the math in each scenario because the math differs based on a few things like Fico score, CLTV, Loan size, and time frame between purchase and refinance of new property.

You will need to read them and do some math, but this would allow you to calculate costs and evenly allocate them if they go 50/50.

I think that's entirely up to you and your math.

Need numbers on this one to do actual math.

I need this explained in layman's terms please I am still learning and the math breaks my brain a little, haha.

And folks from the mid west would say 3 to 4k  and some would say 5k with most folks thinking these days 10k is enough.. and then just as you say work the math backwards at 300 a month which is pretty tough to make if your max leverage and you have your answer of how many houses you have to own and manage.