Can be great. If you have spaces between teeth or under bridges or around implants, proxabrushes can be an excellent aid in cleaning. See your hygienist with them to learn how best to use them in your specific areas. Good Luck.
Yes. Especially if you wear braces or if you have hard to reach areas.
Possible. If you have recesson and spaces between your teeth or you have braces, proxi-brushes will help aid in cleaning those areas. If you have otherwise healthy gums and minimal recession then you may not need them. They has special use. See your dentist for more answers.
Yes. Good oral hygiene is primarly the use of flouride contained in the toothpaste, you may use your index finger too for brushing, remove removable rest of food. Plaques and crusted material inbetween tooth and gum needs the dentest brush.