If you do the math, there is really not that much coolant in the system when the cells are in the standard close packed honey comb arrangement, line dead space actually is a significant percent of volume. I think the dow fluid is really nice, I've used it on other non automotive projects. has a variety of tunable boiling points, is insulating, ect. Just really hard to seal, but that's the design fun. I think o-rings on the cells is a last resort, or you design it to accept some weeping at the seals. From my "bucket" tests 18650s seem to perform just fine submerged, but I am early on in all of this. I am really hard after the mechanics of the car, as battery technology seems to move faster than the mechanics.Yes...o-ring. Several thousand of them. None will leak.
The coolant is heavy & not cheap. Then again, if you o-ring, might get away with water/glycol?
Note that your cell fuse is also cooled, lol