1) Will there still be critical bugfixes and security updates for 5.6? And how long will this continue?
2) Will there be a migration script for 5.6.* sites? When answer is yes, then when will it be released?
core team might answer, but to my knowledge based on forum activity elsewhere;
1) Yes, i've read for about a year at least, where needed.
2) Don't count on it. There might come an export/import solution, but no regular update option.
2) I'm not sure there will ever be something to completely migrate your site, there are far to many core changes to be able to encompass everything. Not to mention that any plugins you were using, may not exist in the new 5.7 world so anything leveraging them would inherently be broken. That said, there may be some tools to help later on down the road, maybe some things to import content to a generic area on pages or something, I dunno. Anyway here are some threads with MAYBE some answers you were looking for on this (including one from AEmbler)
Migration thinking and tools for importing content into 5.7 will come in time.
Having said that, it also depends on the community. I barely have time to test every pull request and fix every bug myself. Right now it's mostly mlocati who's helping out, but if more people would help with 5.6 fixes and not just complain if something's broken, I'm going to manage it for longer. I'm sure you can all understand that taking care of something isn't fun if people just complain and never help.
Luckily, things have gotten a big quieter now that 5.7 is out of the door. 5.6 is pretty stable and hopefully will require less and less of my time. I'm not making any promises, but I believe that 5.6 will be supported for more than a year..