Practice is the only real way to master anything. Or at the least get a handle on it.
1. I don't entirely get what you're saying, all I can figure is, you're either spining out of control by any
and/or of the following, too much throttle and/or too little countersteer and/or going too fast and/or going too
slow (causing spinouts) or you're understeering by either letting the car regain grip quicker than you would want to
by countersteering too early or letting of the throttle too early, or something of the two, or letting the revs drop, etc. The only real
way to nail down a proper drift with any car on any track is to just pratice, and to keep up with it.
2. It's down to what you want, the DS2 to my experience has been easier, but I find it more rewarding to drift with the DFP
as for me, far as grip or drift, it's a tool I've found to concentrait more (least as far as GT4, GT3 is a different story as I'm
very good with the DS2, drift or grip), bu that's mainly on expreince, time and commitment. I've spent the most of it
on GT4 with the DFP, so I would feel it's the better choice, but it's down to what you want and how much you're willing
to train on and whatnot.
3. It matters on one end, not really on another. Most RL 'drift' cars are Nissan Silvias (K or Q, also the 180/200/240SX, S13, S14 and S15
models), a given RX-7 (FC, FD, etc), some BMW's and Corvette's I've found to be driftable (Z06 for the vette and the M3 be it normal,
CSL or GTR, also the M5 which is tail happy to no end and also a car you honeslty have to concentrait to keep on the strait and
narrow on N2's

), I've found I could drift the Ford GT on N3's.
A good car though would be a typical cheap JDM-ish car like some of the mentioned, also the AE86 Trueno, the NSX can also be dritable,
same for the SW20 (MR2) although they are tricky due to the fact they are MR, but more so for the SW20 to my experiene (same for the newer
ZW30), the NSX is well behaved untill you do throw around the weight, and if so, get ready to not spin out into a wall.
But really, almost any car can be good for drfting, it's to what you want, how you set up a car, yada blah yack, but mostly practice, practice, practice, becuase I could tell you just how to do it, but the only real way is to find out for yourself. Try a bone stock car like say an S15 around Trial mountain or Tsukaba, even Suzuka full and just try and send it sideways again, agaian, and again.
My take on it anyway.
Till later.