I don't have weatherstripping on the door panels but since aluminum is such a good conductor I covered my doors with the good bubble Insulation from Charlie's. Their is not room on the inside door needs to open so I put it on the outside. I live in Northern Indiana and it gets cold here so I have mine well insulated.

The door from the garage directly into the house is the most vulnerable access to your house. Once a thief is in the garage, he can work unobserved to access the locks on the entry door. On one house I owned, the garage was attached but didn't access the house directly. You had to exit the garage and then use another door to access the house. I received an insurance discount because there was no direct access to the house from the garage. And while locks cannot prevent all burglaries, you want to make it more difficult for the thief so that they will go someplace else.

Not uncommon. People do it for privacy if the door's line of sight is within a public area. Negative as rrah said, is it makes it difficult to move furniture into the room. But it's a subjective question of privacy preference. It doesn't bug me as long as there's decent enough space to move large furniture. In your example, it depends on what is to the bottom right but I'd consider shrinking the closet a foot

Yes but it is tricky since there are so many variations. Many companies make aftermarket hardware that fit other doors- Emtek is one. You may find it easier to find out who manufactured your doors and buy the hardware directly from them