If I’m reading this right, you have
- multisite.com set up with an Elastic Load Balancer using Route 53 and a wildcard certificate that also includes newsite.com
- newsite.com set up as an A record directly to your origin server not ELB
In which case you’re most of the way to routing all newsite.com traffic through ELB which I think is going to be the best option:
- set up newsite.com on Route 53 too – I think you have to do this so that you can ALIAS the newsite.com (without the www) to your Elastic Load Balancer, since you’re not allowed to CNAME a root domain and you can only ALIAS from inside Route 53
- in your Namecheap DNS console, configure Route 53 as the authoritative DNS servers for newsite.com
It’s not possible to take the AWS SSL certificate and install it on your origin server: Amazon doesn’t give you access to the raw certificates, just serves them for you on ELB and CloudFront.
Or alternatively you could generate a letsencrypt.org certificate for newsite.com for your origin server, which will need renewing every few months but you can set that up automatically. This should be safe since your elastic IP for your origin server isn’t going to change so you can A newsite.com to it as you already have. And would save you the (small) cost of Route 53 for newsite.com. Both method will scale to new domain names too.