There is no need to scan the whole IPv4 space. Completely wasteful. If you already know the CDN, then you can easily find the ASN and all the IP ranges under that AS. Very large organizations may run more than one ASN but you have tools like Robtex to explore further. (BTW an IP range may also be announced by more than one ASN).
The answer is maybe. It depends on how the webserver was configured, in particular if there are remnants of the past (pre-CDN era) in the configuration.
A properly-configured webserver under a CDN should not respond to direct requests made outside of the CDN.
Better yet, the webserver could have ports 80/443 open only for specific IP ranges (those belonging to the CDN), so the ports would appear closed on your end while scanning since your IP address is not among those whitelisted.
My advice would be to read the documentation applicable to that particular CDN, then you can test if the server in question followed all recommendations. The CDN may already be doing a lot but it's important to understand what the CDN does and does not (or cannot do).