Pasteurization in the bottle leads to sediment which is usually not a sought after quality. Probably cold crashing / fining / filtering and sulfites are most commonly used. I would bet all are probably force carbed too.
I would advise using a mix of 50/50 black cherry and tart cherry, and maybe up the honey to raise the OG just a tad if you are backsweetening with juice instead of concentrate to make up for the dilution.
The secret for me is scaling down on malt extract (or even do all-grain! Makes a world of difference.) And scaling up on the crystal malts (I use 1lb 60l).
I'm assuming if he is kegging that he is either A) Keeping it cold enough not to ferment or B) Stablilized with k-meta and sorbate. But you're right, if he didn't do either of those it would still ferment down to dry.
Just the normal tube. 1 can definitely isn't going to make it sweet if it...
He said he was kegging, so that's a moot point. If you were bottling you would have to stop fermentation by stabilizing before or pasteurizing after if you want to carbonate.
Sounds like it was probably going already, shaking it just made the dissolved co2 leave the solution. If you've ever added powdered nutrient to something after fermenting a bit the same thing happens.