I still have both cars and switch back and forth between them a lot (until next week when the 335 is gone).
My 335 is 6MT and my M3 is DCT.
That said to answer your questions:
1.) The M3 is more confident at high speeds than the 335i, more noticeably in the corners than in the straights, but still noticeable in the straights. The steering on the M3 is surprisingly light at low speeds compared to the 335i and the car under-steers less.
2.) The 335 feels that way in 6th at 70 too.. I don't even downshift to pass most of the time. Fact is, above 1200 RPM (1400 on an N55), the turbos are spooled so you're going to have torque. The M3 requires gear changes. If you purchase a DCT, put it it automatic mode for a while to see how the computer controls the gear shifts and you'll start to see things like 7 -> 4 when passing on the highway. I came from an RX8 to the 335 to the M3 and the car wants to be rev'd just like the rotary does and it's extremely rewarding to do so in both engine response and sound.
3.) My 6MT 335i does not feel this way at all. Having never driven an auto 335i, I can't really say I know how you feel, but the 335i is probably the easiest, most effortless manual I've ever driven. I would assume what you're experiencing is due to the torque converter. The smoothness of the M3 is really going to depend on how you have the drivelogic set on the DCT and how you shift it if you're in sequential mode. The nice thing about DCT is you can always just throw it into automatic mode with a mid range shift program on the drivelogic for rush hour and then it's smooth as silk.
2016 Yas Marina Blue F82 M4 ZCP
2011 Le Mans Blue E92 M3
2010 Le Mans Blue E92 335i