I’d like my circumstances to allow for some creativity.

So with many of the circumstances you describe, it's probably all knob-and-tube or, at best, rag wire.

Does anyone know of any issues that we may run into under these circumstances. 

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.