Hi! I'll try to respond to your requests here.
I'm hoping it's OK to ask this type of question here.
Yes of course, we're happy to help with planning questions here.
Now, I'm debating whether I can keep the two post types and add that Parent/Child relationship (Workshops are children of Presenters), or if I could accomplish this creating a User Role of Presenter, and having the Presenters tied to their Workshops as the "author" of their Workshops posts.
You could probably do either, so it's important to understand both approaches to come up with the best solution.
One potential drawback I see for the user role-based approach is that it's not possible in native WordPress to assign more than one author to a post. If your Workshops can have multiple presenters, then this could be a bit of a challenge for you.
One potential problem I see with the Presenter post type is that you would be managing users in more than one place, which can be a pain. You also would lose WordPress's built-in permissions system for authors, editors, contributors, etc., as well as any Access role-based restrictions you may wish to apply.
I would like users to be able to click a Workshop title to see the full description.
Okay so Workshop will be a custom post type, which I think you're already covering. No problem.
In addition, I'm planning on using a tooltip plugin to display a Presenter bio (name, title, headshot and description), so I'm wondering if that info could be pulled in from the User Profile if I went that route instead of from the Presenter parent post.
I assume this can be accomplished by adding the correct markup to your table - that's how most tooltip plugins work. If so, then you can use shortcodes to grab this information from either the Presenter post, or the user profile, depending on how you decide to set this up, and insert it in your table.
The other thing I'd like to do is use CRED to allow the presenters to either create their own profile, or create the Presenter Post, and in either case, be able to come back and edit their Presenter info, as well as create/edit Workshop posts.
Okay sure, CRED can handle these types of forms for you. I think the user profile approach would be more straightforward.
It's not 100% necessary for the Presenter to be logged in to edit this content. Front-end forms wold be fine.
I assume you mean that you don't need the Presenter to log in through wp-admin to be able to update their own profile or Workshop posts. This approach can be accomplished with CRED and Access. Some type of login would be required to restrict access to a user's profile or Workshops, but it wouldn't need to be through the wp-admin area. You can provide your own custom login forms in the front-end of the site.