To what extent will the parties be free to carry on other businesses- while the joint venture subsists- if one party pulls outIs it intended that spouses/partners should also be shareholders, to allow for tax advantages from a broader split of dividends?

After 2 years he will add me as the second shareholder of the company.

After W2ing themselves, they both have another $100k sitting there.Sally gives herself a nice "shareholder distribution" (recall, she owns 100% of the shares) as a Christmas bonus to herself of $80k, leaving $20k in the business for operating expenses going into the new year on December 24th.Johnny decides to leave the funds "in the business," for "future investments."

I would only be worried about it if I were a shareholder in one of those iBuying companies. 

Assuming that you are going to be setting up a C Corp anyway because most tech startups are setup this way if they are going to have a large number of shareholders, the ROBS solution sounds appealing, until you get into the details. 

Its still a pass through entity so you are still paying tax every year on your profits.As a S Corp, you should be holding shareholder meetings (I think quarterly) with meeting minutes and having decision memorandums when you do anything "major." 

Other shareholders participate to a greater extent than Betty.

If it’s your own personal cash, it’s considered a shareholder contribution.

When the corporation pays the earnings out to their shareholders as a dividend it is taxed again (20% top rate).

And to further that, shareholders may not want that either.