The reason slipping is considered more dangerous than other techniques (though not so dangerous as to prohibit) in the glide is that the aircraft is at low speed. The slip can increase stall speed a little, and with increased drag there is a chance of a distracted, busy, inexperienced pilot stalling. Not too much of a problem in balanced flight, stall recovery is relatively easy. If an aircraft stalls while slipping it may well spin.
Good point about min sink then best glide, but the difference is slight in a training aircraft, and we teach an early decision as most PPL flying is at low enough level that the glide will be a busy time. The change in speed simply is another issue to worry about, and I would not consider teaching this to a pre-PPL student.
The base is used to assess the wind amongst other factors. The drift is adequate for this, as a strong wind will blow the aircraft out, giving apparent sink below the normal sight-line angle, and causing the pilot to turn in early.
The problem with a long final is that the aircraft is committed. The glide can be shortened, but never extended. I think that is why you are so enthusiastic about slipping in to allow extension. A glider's airbrakes are designed for this sort of operation, training aircraft have no such advantages. I have never been taught a long final. The stable approach is no good if you find you are too low. The short final/long base allows all the extension/adjustment required.
I can think of no reason I would choose to land downwind after an engine failure that would not leave me with such a great advantage that the bastardisation of the usual circuit would be simple, except perhaps the "only one option" going over vast woodland, that can only be flown the day it happens so is not in the scope of a discussion of training.
Your "10,000-foot concrete" is fine, just fly for it the best you can. Aim 1/3 of the way down, flaps, certain of 1/4 more flaps, certain of 1/8, full flap. You'll land about 1/4 - 1/3 the way down, as the flap only makes difference of 1/3 of a small field, not much in 10,000 ft! However that again cannot be taught: either there is only one way to make it, so fly straight there and select flaps as above, or you're close enough to make some sort of base and final either downwind or into wind, in which case fly as taught in the syllabus. Any other reason for landing downwind must be a last-minute choice, in which case all that can be used is the EFATO techniques.
Important note: the PPL syllabus specifically does not include "turnbacks" - downwind landings from engine failure shortly after take off, similar to your description of gliders losing tows. In the military their practice is banned from all but instrtuctors as they will kill you if you are not very good or do not have plenty of height. They still did killed a few I heard about. Gliders have a steep climb out and a good glide ratio, plus pilots experienced in engine-off landings, so are much safer in this manouever.
Downwind PFLs from the crosswind leg were standard training where I learnt to fly, but you have a whole big field, may have a convenient runway and are in about the right place.