WordPress 301 permanent redirect with exceptions

I’d like to set a permanent redirect whenever someone visits myoldsite.com or its subpages to mynewsite.com like that:

visit myoldsite.com go to mynewsite.com

visit myoldsite.com/subpage go to mynewsite.com (and NOT mynewsite.com/subpage)

I would also like to make exceptions to the above for wp-admin and wp-login subpages. I don’t have server access to myoldsite WordPress installation, so I can edit .htaccess file only through a WordPress plugin. I would also like to make an exception for .htaccess if possible, in order to be able to reedit .htaccess via my ‘WordPress .htaccess plugin’ once the redirect is done, so I can have the power to revert things back to default if necessary.