1) can you cancel a launch half way through to its destination?
No. Once launched it's on it's own.
2) Also, can you switch subs from attacking to a gift part way through a launch?
Yes.
You can also gift a sub when it's on its journey (a handy trick. Eg. you send subs at an outpost to appear you're attacking, but when out of sonar range you turn it into a gift)
It's important to point out a gift cannot be turned back into an attacking sub.
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