(1) is relatively simple. You need to use the $datalist_options config setting. All that needs to be done is at the beginning of edit_entry.php to poplulate $datalist_options['entry.organiser'] with an array of users. MRBS will do the rest.

(2) is trickier. You'd have to use some JavaScript because you'll need to be able to change the email address if the organiser is changed.