Smoked porter FG and carb issues

I brewed a smoked porter and have 2 issues with it--it did not carb naturally in the keg, and its gravity is still high. The story is I brewed it, let it ferment for 3 weeks (didn't record temp in my freezer, but should have been between 62-65 the whole time). OG at 1.074. FG came in around 1.030. I used 1 pack of S-04, rehydrated, which may be my problem.

So I kegged, and then got a bourbon barrel so I stuck it in the bourbon barrel for two weeks, then transferred back to a keg and primed with priming sugar. Another 3 weeks in the keg. I hope the sugar plus add'l time drops the gravity a little bit. I put it on tap tonight and it is basically flat. It still seems to be around 1.030 tho I'm letting the hydrometer sit out overnight to see if there are any bubbles I'm missing.

It's force carbing now, but I'm thinking of pitching a small starter of either WLP-002 or Burton that I have washed, with the thought that I might be able to get the gravity down a bit more.

The Question:

Is it ok to pitch this in the keg? I'm thinking about just venting every day or two to relieve any additional pressure. Or maybe venting plus leaving it hooked up to the co2 so it can carb and finish out at the same time? Thoughts appreciated.

It is tasting pretty good but I'd like to get it finished!

realized--obviously it won't ferment under co2 in the fridge b/c it's too cold. any way to calculate how much carbonation I'll end up with if it drops from 1.030 down to 1.020 or maybe 1.015 (or any way to calculate how much it will drop from 1.030 with a 500ml starter?)