I put a hastily dug up JC yama back in native soil on my property in Sept, no brown yet it’s almost January.
They can live around six months with no roots. My record is 10 months with communis cuttings.
Keep in mind that it's winter, and that it's unlikely to go brown during dormancy because it's not using any water. Junipers can die without going brown as well. I'm trying to coin the term "Grey death" for those cases.
As for the sand.. It's not magic material that opposes gravity and it can and will sink to the bottom of your pot and clog it. Sand holds water very well if it's surrounded by pot walls yet it can dry in a matter of hours after which it either turns to dust or becomes rock hard and water can't penetrate it. Cool for seedlings and I'll happily add some sand to my pots for moss to and a lil bit of silicium.
Pines and junipers on the succession scale are "first colonizers" they don't love sand, they're not happy to be in sand, but they tolerate sand better than the other plant species which is why we see them there and why they get to grow old there.
If you'd be able to grow clones of each in both sand and organic soil, the organic soil always wins.
If you keep digging, you'll see that a bunch of those pines have serious tap roots that dig down deep, with no feeder roots anywhere close to the soil surface unless there's an organic layer near the base. That's why we never collect in sand. Those are trees to look at ;-)
Show me an inch of new growth on your communis and I'll happily crown you the communis guru though! If you find something we haven't thought of and it works, it's absolutely valuable!