Real quick, how long did it take to tweak the original Starcraft Campaigns into Co-op, multiplayer levels to play in battle.net? what was the process like? Was it a lot of effort or not too difficult?

I wasn't involved in the making, nor did I follow the progress, but having good mapping experience I can imagine how it went.

First you extract the maps from the mpqs. There's tools for that. - the quickest step

Then you open each map and add another human player and alter the map layout and resources to accommodate the change. - not too much work if you have some experience

Then you modify the triggers to play cutscenes and distribute units where applicable. - another step of medium effort and difficulty

Finally comes the hardest and most tedious part: Testing and balancing.

All in all probably a couple of days of work per map, but nothing that requires too much mapping skill.

I made my own co-op maps out of the original campaign (and some fan-made campaigns that I never released publicly). It was never a difficult excursion beyond testing.

Wait a minute! "alter the map layout and resources to accommodate the change?" I can see how that goes too. Also, it sounds like you had to also remove every unit and replace them with the new ownership. God, that sounds like a lot of effort... To me, at least.

But all in all, this project probably took a couple months to complete, if it was one, right?

Doesn't sound like that to me tbh. Most of the time the player has few starting units and you can easily copy and paste them for another player in bulk. Or just cut the starting units in half, mark them and bulk change the owner property. Done in a couple of clicks. It's more work to rework the terrain to make room for a second base and add an expansion or two for the other player.

How long it actually took is a complete guess, unless you actually know, because there so many factors that have tremendous influence: If you half ass the balancing phase you can save a ton of time. You can either map a few hours a week or many hours a day, you can do it in teamwork or alone, you can either try to be very tedious with the details or just keep the changes to the minumum ...