Do CNAMES still work once you change your nameservers back to regular ns1.hostingwebsite.com? Does the domain have to be parked/masked/forwarded for the CNAMES to work properly?

Nope. Once you change the DNS to the hosting site / your own web server all other records get remove once the DNS get propagate

Sometimes you have to remove all other records first and then change the DNS

Maybe you should wait for sometime and then try.

So how would one check his email if the domain needs CNAME changes? Can i do it through the host and leave the domain alone?

That's if the hosting company will allow it.

Why do you need CNAME records to check email?

i'm not sure that i understood what you mean, but you can create the same records on your current DNS at ns1.hostingwebsite.com and then swtich the nameservers back

good luck :)