Well, on the desingh, i recommend you to use illustrator, but if you want something simple, go ahead and use captivate, but going further, the best option to generate a certificate, its using a LMS applying a certification plugin.

Absolutely. You can design any sort of certificate you like and set it as a background on a slide and have it fill in as little or as much variable information as you want for the learner.

Can you have a look at the Learning Interaction ‘Certificate’? Try it out, and for more questions, post back here. Some fields are filled in automatically. That is the case for the date, the name of the course (if you have it under Project Info), the name of the learner (if it can be retrieved from the LMS account) etc…