I was a construction electrician and I ran a lot of conduit before I found a job that is a lot less work.
Anyway, I remember that the NEC allowed a splice in a condulet if it satisfied the fill capacity and had at least three openings, not including the cover, of course.
Do I remember accurately, and is it still the case? A coworker went a little ape shit when I told him this. He’s not an electrician.
If he’s right I’ll go ahead and put steak on his black eye. 
Peace,
mangeorge
Are we talking plastic or steel?
Makes a difference in terms of earth bonding.
Here is a link to a discussion of exactly this with NEC cites:
Basically it looks like you are correct. The condulet needs to be marked with it’s volume as well.
ETA: there is also the suggestion to upsize the condulet using reducer bushings to allow more room for the splice.
Metal.
That’s how we used to do it, with a plug in the unused hub.