What happens if iPhoto is opened and the volume with the library is not available?
To answer your first question, nothing really bad will happen when it can't find the library. Basically, if it can't find it, it will prompt you with which one to open. This would be the same as holding down Option when launching iPhoto (or iTunes, Aperture, etc). 

The below folder was after I had created a new library on an external drive, and then disconnected the drive. When I plugged it back in, this list had a new option saying 'Test' (my test library), and default beside that. 



So, all iPhoto is doing here is asking you if you want to open another library (because it can't find your default one). While not a benefit in iPhoto, aperture actually lets your export a project as a library, so you could take only a small part of your library with you on the go to work on, then re-add it back to your library when 'connected' again (I don't think iPhoto does this, but I am not sure, I use Aperture more than iPhoto). 

For your second question, yes, the other MacBook Pro should be able to see it as well. In that case, you may want to hold down Option to select it from this drive, etc. 

The only thing you need to be worried about in that case is not having it open on two machines at the same time (if it was on a network drive). It may prevent you from doing this in the first place, but if it doesn't, it may not be supported and may have issues.