Is there a way to mention/tag a user in a question or answer?
If you just want to mention a user in your question, you should link to their profile.
If you're trying to get a specific user to answer your question, that's not a feature of Stack Overflow. Ask the question of the whole community. If the person is an expert in the technology you're asking about (and an active SO visitor), they probably follow the tag and will find your question. It's entirely up to them whether they answer or not.
I'd like to use mentions to give credit to others' suggestions to my answers. Having to get/make a link to their profile and add it as an explicit link in my response makes it tedious.
If you only want to mention/credit someone in a post without notification, then yes, it would be nice to just @mention them. The problem is that display names on Stack Overflow are not unique, as they are on many social media platforms. I'd have to go to your profile to get your unique user ID anyway, so at that point I might as well just make a link.
If you want the attention of a specific member, you can look for them in the chat - you can see the active users list in the following links: (more about the reasoning behind the separation to three chat domains can be found in Why is Stack Overflow on its own chat network?).
Another option is to check their profile page; many have websites and even email addresses mentioned there, so use that to contact them.
If I understand this correctly, then what you are looking for is to ask/invite some specific users to check your question and answer them.
You will not be able to do that in your question. Such feature is not available in SE sites.
however there is a kind of work-around to get the user's attention to your question. i.e. You can add a comment to your question using
@<username> which will send an alert/notification to that particular user.