Fwiw you can change ssl settings for a single subdomain using page rules or configuration rules
But that aside I would always recommend using whatever ensures encryption all the way through, which would be Full strict and a certificate on the origin. Setting to Full would mean the connection between CF and origin isnt secure, and unproxying means you lose all the benefits of the CF proxy (Performance, security, etc.) The actual choice is up to you
That returned error means they couldnt reach your origin which means none of your websites code would be there it would entirely be a page returned by cloudflare
Unmetered Rate Limiting is free now, throwing it on might not be a bad idea (although if it is truly distributed it might not do too much). I would make sure they aren't bypassing Cloudflare either (you properly have only allowed CF IPs to connect)
(its not the same as Advanced Rate Limiting, thats still the name for the ent addon that has far more options, the new one for paygo is generally called Unmetered Rate Limiting)