how do I reload Apple Mail

Apple's instructions to Choose Mailbox > Rebuild could be worded a little better. What that means is to select (single-click) the Mailbox and then choose the Mailbox menu > Rebuild.

Like so:

Select the Mailbox in the Sidebar first.

I happen to have a Pictures folder "On My Mac" selected in that screenshot, but you can choose any folder in the Sidebar to rebuild.

---

Before you drag the iMac to your local Apple Store, contact Apple first using this link: Official Apple Support. They will almost certainly have additional suggestions.

There is lots of life left in any Mac that can run Big Sur. It's not ready for the recycle bin yet.

I understand you are unable to save an email message in a local folder created "On My Mac" in accordance with Create or delete mailboxes in Mail on Mac - Apple Support. Is that correct? If not, please explain what you are attempting to do instead.

You can rebuild Mailboxes in accordance with Rebuild mailboxes in Mail on Mac - Apple Support. Try that. You can rebuild any Mailbox, including local, "On My Mac" Mailboxes.

Another potentially relevant Apple Support document is If emails appear to be missing in Mail on Mac - Apple Support.

I realize I did not address your initial question which is how to reinstall the Mail app. That cannot be performed separately, but you can reinstall macOS: Reinstall macOS - Apple Support. That remains an option but try rebuilding those mailboxes first.

In fact I'd rebuild all Mailboxes including All Sent, etc.