I had to dispute it to gain an additional 6k on the appraisal.

Your mortgage, your equity and your gain do not matter.

I have nothing to gain by you investing in your market.  

So, if the value goes up 2% you gain $20K on a $500K in equity or +4%.

We are going through a contentious divorce and I would like to buy her out, but apparently thats not very easy.If we sell the properties our gross earnings will be about 300k, while our net earnings will be around 150k, due to capital gains, depreciation taxes and realtor fees.My lawyer, who is not experienced in real estate investing, says I have to buy her out based on gross earnings, so I'd owe her 150k.

If I were to sell and build the two smaller houses, I could likely fund them enough from my gains on the current rental (and a little out of pocket) so they also cashflow ~$300-400/mo each.So, at the end of the day It’d be a wash or a little loss, cashflow wise, on one house and I’d gain ~$300-400/mo on the other house.

Does STR income after the first year become passive income? 

My question is, would I owe the full 20% capital gains tax on that amount?

Whatever you think you're losing on Airbnb through fees, you're likely gaining by having a consistent pool of potential renters.

There is also no target right in terms of an asset, and you need that.That's a fair point, I don't want to get in over my head but I would like to get some passive income rolling...I just need to know if there's any actual feasible way to get me started in this business at my income level.