1) I think you should read up a bit more on the BRRRR concept as I don't quite understand how you are trying to use it in this situation. In a nutshell, it states you buy a property, rehab it and then use the new equity created as the down payment when you refinance this have little to not money out of pocket in the deal.
2) I would speak with a lender as I am not sure about this but IMO if you go ahead and split the lot and it has an appraised value of around 100k, that gives you more "equity" if you were to get a HELOC on the combined property. For example, it would show you own your house valued around 400k with about 200k in equity as well as you own a lot valued around 100k that is free and clear giving you an additional 100k in equity. Whether lenders will loan against lot values I have no idea but to me it seems to make you look better on paper.
That said, it seems like you could leave everything alone currently and still have enough equity in your home to use as a down payment on a rental. So if you truly want to stay put and not build then that is the direction that I would go. Hope this helps.