Under the circumstances the options should be either to close or default.

I'm not a tax advisor (see your own personal tax advisor/CPA), but don't forget to take into account the new tax law is not supposed to allow for deductions for HELOC's any more save for a few circumstances.

Are there circumstances where some investors might choose to use separate bank accounts and credit cards for each Series? 

Read this:  https://www.fool.com/investing/general/2015/10/02/...Under certain circumstances, you can roll-over your 401k to a self-directed IRA and buy a rental property from your IRA.1.

I'm saying it's 25/75 per a private agreement and how my parents have set it up in their will as well but for all intents and purposes, could we treat this as a 50/50 arrangement at least when it comes time to sell under the same circumstance?)

@Landon Belk answers to your question will be different depending on many circumstances.

I am considering keeping my policy and not allowing co-signers/guarantors under any circumstances and just keep looking until I find a better tenant to meet my standards on their own.

Some smaller deals may even be financed with seller finance only or by assuming the existing loan on the property due to a number of circumstances.

In the right circumstance these can be very helpful.An improvement exchange has the QI taking title to the new property before you do. 

Best to talk to your CPA/EA who knows your exact facts and circumstances and can lay out all the options.