How do I import to iPhone from Editor

[dflat]

...Then I chose "Email Selected Song to iOS Device." A new-message window appeared in Mail. But instead of a link it had the text (html) version.

I presume you are using the latest version of Mac editor? (v0.6.4.) When it appears in Mail it will show the code. Once sent then collected on your iPod touch, tapping on it in Mail should open and import the song into iReal book.
But if you are using a gmail address for this, then this could be causing the corruption.

...I'm not sure how I'd "disable" the Mail account, though I can use Mail's own prefs to make it not the default email reader.

On your computer; Mail>Preferences>Accounts>Advanced,
you will find the Enable this account box.

[*] This method has another disadvantage: during the five minutes while Mail was the default email app I got two messages, both of which ended up permanently shunted away from the Gmail I need to keep my mail in.

You can email them back to yourself just so they are in the correct place (but remember not to directly reply to them as they have been forwarded to yourself.)
On your computer; Mail>Preferences>Accounts>Advanced,
you will see the option to 'Remove the copy from the server after retrieving a message' ... you can turn this off also.

I am still wondering if you are using gmail for your email and if you have another choice? as this could be causing the problem.