1-2/ I'm pretty sure the procureur can be named from anywhere , to any court, an can be an ex-arttorney while I think the juge d'instruction is named by the local magistrates, but have to have followed a very strict formation beforehand (and I think you have to be a top scorer or something).
3/ judges are chosen by passing a very competitive test before 31yo, then being taught during 3 more years (to access the test you need a master degree, if you fail you can still become an attorney, and attorneys can work a number of years and pass the test again, but in that case the age limit is like 40)
4/ here I'm not exactly sure. I go to the police, they investigate. If they found nothing but have reasonable suspicions, they ask a judge to authorise a 'perquisition' (basically a warrant, which have to be served between 8 AM and 10 PM). A juge d'instruction will only be responsible for an investigation in very difficult administrative cases (they are insulated from political pressure as much as possible, that makes them very good to investigate corruption cases) or in a heinous crime (basically anything involving children, multiple murders, terrorism)