The "beat" here refers to the heartbeat in the previous clause. It means "when the heartbeat stops", or, when the President dies.

It's interchangeable with "for the moment". To be honest I hardly ever hear "for the present", and even then only for some official/news reports, not for day-to-day life.

"Here look" is used when drawing someone's attention to something such as a completed task or something interesting, especially after another person had inquired about said task or thing. "Did you write the correct address on the form?" "Of course. Here, look. There it is." "Where's the dog food?" "I put it in the cupboard. Here, look! Just where I said." You can also substitute "Here look" with "See?" "Did you finish your homework?" "I finished it an hour ago. See?" To call someone's attention in general, such as for a conversation, it would be unnatural. It would be proper to say "excuse me" or to use their name.