How to sign out of iCloud on iPhone and Macbook without having access to rescue email and with Find my iPhone and Find my Mac turned on?
First of all, the comment by Patrix is an excellent way to start. I've helped many users by using the various options there (sometimes having to add other security email addresses, calling Apple Support, etc). This process may or may not be extremely frustrating - I've seen users sort things out pretty quickly, and others who have had to wait days for Apple to call them back once everything was sorted.

However, another option which may or may not be palatable to you is to just contact your previous employer. 

About six weeks ago I came across a very similar situation, except in their case it was an old email address from an ISP they were no longer with. After some negotiation the old ISP agreed to recreate the email address at their end for one hour and set it up to autoforward to the user's current email address. During that hour the user was able to access the emails from iforgot.apple.com and change their email address etc.

Of course, in your case it may be a case of being allowed access to your old email (either because it still exists, or because they've recreated the address for you). Or they could set up an autoforward as happened in the above example.

If this isn't an option, then iforgot.apple.com really is your only option.