For call to action (CTA) buttons and links in emails, there are two perspectives I would consider:
Ease of use
Usually users will seek the easiest way, hence click on a hyperlink where possible instead of triplet select-copy-paste.
Transparency & Trust
On the other side an explicitly made transparent URL could convey a sense of security. Some users may ...
Yes they should be able to.
Most confirmation emails, much like rest password email, are valid for 24 hours.
If you don't confirm it then your account is not verified.
"Then, some guy registers using that same email." After 24 hours that email has become available again.