I have added an subdomain to the DNS record, and would like to use the free SSL certificate in FULL encryption mode (flexible mode has caused content distorted in my wordpress).
Since the destination server does not enable port 443 (as no SSL certificate installed), how can I redirect the pointing from port 443 to port 80?
You should neither use Full nor Flexible, both keep your site insecure. You should use Full Strict.
You need to install a certificate on your server - use the search here for details - and once your site loads fine it should also work fine on Cloudflare. Keep Cloudflare paused until it works fine on HTTPS.
We are using shared hosting so the installation of certificate on the server is not possible… Therefore I would like to use the certificate by cloudflare, but need cloudflare to point to port 80 instead of port 443…
Cloudflare is not to provide SSL, you need a proper certificate on your server. But there are plenty of free certificates out there. If your host does not support that you either pay them or switch host.