That seems to have solved the problem. Sorry to make you wait so long.
I'm afraid it's still not working. Even when it declares the value as 1, the egg still doesn't follow the snowball's path, no matter which order. I'm still not sure if I'm doing something wrong, or there's a legitimate bug. I'm going to post up some pictures of the command block in question.
Thanks for helping @tryashtar , but it doesn't seem to have worked. I jumped the scale up to 100 in the second set of numbers, and 10 in the first set. The behavior's no longer erratic, but the projectile's still not following the snowball's motion.
/execute store result entity @e[type=egg,limit=1] Motion[0] double 10 run data get entity @e[type=snowball,limit=1] Motion[0] 100
/execute store result entity @e[type=egg,limit=1] Motion[1] double 10 run data get entity @e[type=snowball,limit=1] Motion[1] 100
/execute store result entity @e[type=egg,limit=1] Motion[2] double 10 run data get entity @e[type=snowball,limit=1] Motion[2] 100
Shall I update the information, or let the issue close?
Can confirm for MC 17w46a.
A little bit of Information on that in the error messages would have been nice.