I’m very bullish on Passive Investing and creating multiple streams of income (mailbox money as some would say) and see this route as an effective way to achieve that. 

If I HAD to pick one of your 3 options, I'd probably go with option 3 because it's likely the one that you can achieve the quickest... especially if you aren't concerned with cash flow. 

However, it does require solid expertise in digital marketing to achieve positive results.As for list pulling: you've got solid services like PropStream and Listsource to name a few

If a bedroom has two (or more) separate windows; one functions as intended but the other does not... the functionality/egress requirement is achieved via the functioning window, correct?

Of course, this is just my opinion but I bet I'm not alone..If I'm an investor, I think the returns you're able to achieve in Costa Rica are higher than normal because you focus on these large properties and, as @Martin D. mentioned, prices in that country are too high and returns too low.

Nothing else on your list matters if this purchase doesn't achieve your investment goals.Also, you have NO WAY to determine your offer price until you run the numbers!

We reach agreement in principle with the seller on what we mutually want to achieve. 

You have to determine what your goals are and then attempt to achieve them.

If you buy properties in a city where rent increases don't outpace inflation, you can't achieve financial freedom—no matter how many properties you acquire.For example, suppose you buy enough properties to generate $5,000/Mo, which replaces your current income.

cashflow in residential is typically achieved either through a more difficult method, or after 10+ years.