Some interesting findings after moving all DNS settings back to the registrar for the sole intent of activating their email-forwarding feature. This is a head-scratcher: @Jonathan5 I did remove that standalone MX record to see what would happen. The answer is confusing: though everything still works, there is now a 12-ish minute delay for direct-delivery through email, to posts/comments in the forum. Can anyone make sense of why this is? (my forum is not live yet–still testing/setting it up)
I also learned this: I can’t use the above-linked “feature” and also add an MX record. If an MX record is added, in NameCheap, the email-forwarding feature is disabled and the whole area you see under #6 in that link, is grayed-out and without any content.
What exactly does that MX record do? Mine goes to mail.mydomain.com. By leaving that out, am I introducing a new problem? Seems my options are to (a) leave the MX record as defined in the OP OMITTED in order to (b) establish admin@mydomain.org and forward it, for free, to a newly-created gmail inbox, or (c ) forego direct-delivery altogether and instead select some sort of POP/IMAP setup?
To the latter, $8 for the registrar’s “private email service” is dirt cheap. Not sure what that gains me though. If it’s the ability to assign an MX record again, and by doing that, emails are more secure or deliver more quickly, I’d probably do it. Still trying to get the hang of this stuff 
Thanks y’all for being so patient with me, and especially for helping.