One fundamental requirement that the optimiser has is that plans must be safe for reuse, since plans are cached and reused. So any plan generated for that query MUST return...

Qualify the column name.

UPDATE s
SET ID_Request = r.ID_Request
FROM Student s
INNER JOIN Request r ON s.ID_Request = r.ID_Request
WHERE...

???
Don't understand the question

If you're designing a new table, yes. If you're considering altering an existing table, then, as I said earlier