@Jonathan5 Regret to inform my good-luck run has since changed–without me actually doing anything at all. Must have been propagation delays in the interwebs 
. My previous comment was about 10-15 mins after testing the deletion of the MX record. It has now been 90 minutes and the following no longer work: direct delivery, read receipts, bounced, rejected lists etc. Joy.
@pfaffman here’s the skinny on registrar NameCheap. They use an either-or approach: a customer can choose to install as many MX records as they wish in the DNS settings, but as soon as ONE single MX record is added, they no longer allow the customer to create email addresses/alias i.e. admin@newdomain.com to forward-on to another inbox i.e. @gmail.
Thoughts on best path going forward most welcome. As I see it, to make direct delivery operate, I need an MX record for that, as defined by OP here, and another MX–this one, using a subdomain, for a to-be-purchased email provider. Am I on the right track here? Thanks all!