iirc, custom DNS at CF refers to using your own subdomains as name servers for CF instead of the regular *.ns.cloudflare.com. Basically a form of whitelabelling.
Specifying 3rd party name servers as your domain’s name server was (still is?) not possible with Cloudflare Registrar.
Actually, you can assign (not transfer) IP ranges and the one advertising it does not need to own it, but would need a Letter of Authorization (LoA) or similar.
Google now has a publicly available ACME-compatible CA. For CF’s Universal SSL (default/free) Cloudflare may use Google’s CA. But may choose other providers. For Advanced Certificate Manager (paid addon), you can choose Google or other CAs for Cloudflare.
But basically Google would be the one that is issuing an SSL cert for a website. Same as LetsEncrypt.
Swapfiets builds their own bikes and does not - never ever - sell them. If one gets stolen, it can‘t be resold without one noticing it’s stolen, as these are not in circulation in the first place and still owned by the company that built them
The cheapest over time will be .de with as less as 4€ a year depending on the registrar. The cheapest first year domain used to be .xyz with 0.99$ at Namecheap
For CF, we send out these status pages manually. It's usually delayed due to wording, but not to tamper with the SLA. We do keep track of the accurate times AFAIK.