if you screen got broken inner, it will not waterproof. So you need go to a repair shop and fix your screen. since you are wondering after change a new screen will still waterproof, i think yes. for mostly screen repair, they will put a new iphone 7 plus waterproof sticker, so that your iphone will still "waterproof" . Maybe you can call a nearby repair shop and confirm with them.
no, ipad mini 1/2 is same digitizer but ipad mini 3 and mini 4 is different, ipad mini 3 you can change a digitizer by yourself easily, ipad mini 4 you can't, you normally need a lcd digitizer assembly for ipad mini 4, only if you are a refurbished factory have special cut glass tools then you can change ipad mini 4 digitizer.
long press, the message you want save, then it will show copy, copy and paste it to your phone memorandum, then all record you can save like this, normally what's app will save your message for a long time too, so you no need to worried.