1

If a Sequence is designed to vend only once, and Iterator is a class type, can makeIterator() return the same instance every time?

2

I don't see a reason why not.
In fact it may be a good idea to add some such sequences to the Swift tests to ensure sequence operations do not mistakenly assume repeat iteration, if such tests do not already exist.

3

Section Using Multiple Iterators seems to hint that doing so for a destructive sequence is an undefined behaviour.