Is there a difference between 他这是 and 他就是?
I only use 這是在 when I actually see it. Like "他們這是在做什麼?". But for the similar "他們是在做什麼?" without the 這, you can use it regardless of the place or time of the object.

For 就是在, it just basically means "It IS" or "They ARE", when you emphasize the aux verb.