The biggest reason not to practice this way for self defense shooting is retention. That'll leave you with 2 or 3 fingers, depending on the gun, loosely gripping your frame while you cock the hammer.
Or if you're going for a 2 handed grip at the time of cocking the hammer, that'll leave you with your thumb in the way of the slide, either causing you to shoot hastily and injure your thumb, or wait till you have a proper grip to fire. Meanwhile you could have already had a shot off.
And what if you're still pointing at your leg as you're drawing, being in the process of cocking the hammer but not having totally cleared your holster, and then you end up fighting to retain your gun from a grab? Will your trigger finger not get involved and potentially grab the trigger and pull a Tex Grebner? I do believe it could happen, and that's why I don't practice this way with my Sig, nor do I suggest you do.
1911's and other guns with thumb safeties are set up so that you can grip the gun convulsively, as you would in a fight, as you deactivate the safety. There is a reason that they're by where your thumb naturally grips, and not on the back of the slide.
Awful lot of assumptions there... let me add one more to them: If you are close enough to your foe for the possibility of a "grab" while you are trying to draw and fire, you're already neck-deep in the doo-doo, and should be working out some method of putting some kind of distance between you and the enemy-even if just arm's -length, for a moment.
As to thumbing hammer while drawing, in general, it can be mastered with practice, like anything else. As can the DA/SA transition of trigger pulls. Accuracy at the point-blank ranges of your scenarios above isnt likely to be much of an issue-One should hope. If you cant reasonably accurately put 1-2 rounds into someone at arm's-length or less, you should probably consider sticking to knives, or hand-to-hand.
As to the DA/SA transition, believe me, when the time comes, and the adreniline is pumping- you wont be having the touch-sensitivity or manual dexterity, usually, to notice the difference. Fine motor skills just arent as fine , under duress, as they are at the local range poking holes in paper.
Come to think of it- 1st chance I get when back home/ time and weather permitting, I'll try to make and post a vid demonstrating, draw/DA 1st round- SA second round(with a .45) . It's really not that difficult to do well, and with reasonable accuracy, if you include such a drill into your training routines.