There are about 20 CPAs and accountants on this site that specialize in real estate taxation.

Unfortunately it seems impossible to completely eliminate double taxation on the Canadian or US side, especially when the property is sold, and I was thinking how convenient US LLCs are.

Thousands of businesses are funded with this approach each year, so it's pretty popular.Two pluses:(1) The C-Corp can potentially borrow, using the assets that were transferred to the C-Corp from the 401k as collateral, and with your mother and/or yourself providing a personal guaranty.(2) Your C-Corp won't incur the punitive UBIT tax that your SDIRA or SD401k would face engaging in an active business such as flipping.A negative with any C-Corp is the potential for double taxation, but there are several ways to minimize this.

Heroes Act is an example of taxation without representation cause the last time I checked there was no groundswell of landlords screaming for government to help tenants not pay them.

Wouldn't we qualify for the exception  @Brandon Hall discussed as Myth #5 in his article '7 Common Myths About Rental Property Taxation-Dispelled?

For me, while appreciation eats away at the future earnings, i'd need to actively work in order to not only make up the gap i lost in the extra taxation, but, i'd also need to beat inflation in order for it to make sense.

And LLCs faced double taxation while it was said that LLPs wouldn't.  

I will take a course for personal income tax at a Jr. college, and plan to do some self study to educate myself about maximazing my tax shield.

.…...if I am using a Self Directed 401K to purchase a multifamily unit, I know my income is excluded from taxation on a personal level but am I still subject to a business income tax? 

Their logic of income tax sheltering and tax-advantaged appreciation for them isn't wrong, and it makes sense for them given just how much of their income they currently lose to Uncle Sam, but that might not be right for you.That's a very different set of criteria than someone looking for cashflow, someone that isn't already very high income, someone looking to use RE to build income and wealth rather than merely using it to shelter existing income and wealth from taxation Trump-style ("I pay very little in taxes" says the billionaire).