I have 5 folders currently, and the Mini is my favorite. To address your questions specifically:

1) After you get used to the initial adjustment to 16" wheels, the ride is very stable, especially with the Big Apples. I can even ride hands free for about 5 seconds at a time.

2) Yes, Big Apples can fit. I have them. I like them a lot and recommend them. It makes the Mini ride very comfortably without sacrificing speed.

3) I've used seat post mounted fender in the back. Don't know about front.

4) The Mini (esp with Big Apples), and a extended stem provide me (at 6' tall) with a very comfy ride. Oh, I fortified the seatpost and extend past the minimum insertion line by an inch. The combination gives me a very comfortable ride. I plan to do a metric century (60miles) on this bike sometime soon. I have every confidence in the gear range, bike sturdiness and comfort of this bike.

5) The frame and handlebar post feels very solid. That is one feature of all the DTs (I have owned every DT model) that I like a lot.

6) No problems with removing the rear tire if you know how to use a crescent wrench.

Bottom line, I'm a big fan of the Mini. There are many fans here also. Have you read the big Mini thread?