You’re simply using some of the money you’ve already saved for your purchase to show the seller you’re committed and serious about buying their house.

Quote from @Jordan Kaylor: I'm new to real estate investing and have secured a HELOC on my existing home.  

You have acquired skills that are worth 500k, so maybe you can translate that into an adjacent career path and leverage your existing skills?

It also includes managing your existing investment well to ensure you aren't bleeding money due to bad tenants, excess maintenance, etc. 

Right now, I’m seeing max LTVs closer to 80% on purchases, though before COVID, we sometimes went up to 85%.Also, keep an eye out for lenders who promise amazing terms but demand large commitment fees upfront.

Commit* 

The mental gymnastics I'm going through is how exactly that would work without committing fraud.

This will give you the chance to see how well you work together before committing to more overhead.

This approach undermines the trust that should exist in a lending relationship.

Agreed 100%  Lender remedy to call the loans already exists  end of story in my mind.