1. DO NOT increase approach speed for gusty crosswinds - the POH value is the only one to use. Far too many airliner-drivers (who don't really understand the difference between large aircraft with high inertia and small aircraft with low inertia) have imported their 'big aircraft' ideas into PPL-level flight training.
When we conducted a review of the 'Club' PA28 checklists some years ago, we found that someone had said 'add 5 knots if 3 poB', someone else had suggested adding 1/3 the wind if that was above 15kts and, worst of all, they were teaching an approach speed which was 10 kts above the speed which they thought was the so-called 'threshold speed'. So, with a 15G20 headwind, with 3 PoB, people were flying at 63 + 7 + 5 + 10 = 85KIAS all the way to touchdown.... Or what they hoped would result in a touchdown. The aircraft ballooned, they found themselves 22 knots out of trim and we lost a couple of props and nosewheels.
Since throwing out all the 'old wives' crap - and flying base leg at approach +10 with 2 notches, final at POH approach speed and 3 notches, we haven't had a single problem, tocuh wood.
I will personally throw the next ATCO who I hear tell a student on final approach to "Keep your speed up, one heavy behind you at (whatever) miles" off the roof of the control tower!
2. The 'optimum roll control' is another roseate aquatic animal. The only aircraft I've flown where it was sometimes a good idea to use approach flap rather than landing flap in a gusty crosswind was the VC10. Because inexperienced pilots could be caught out by unexpected roll when yawing to align with the centreline - and the inboard flap fairings were quite close to the ground...
3. Teach landings with full flap from the word go. If an FI cannot cope with that, he/she shouldn't be an FI. Mind you, from what my FIE colleagues tell me, that would probably result in very few FIs left - the general standard is pretty low, they say.
4. In the PA28, the only time I would accept a less-than-full-flap landing would be either a deliberate flapless landing, or a glide approach which the student hasn't got quite right - and hasn't been able to select full flap before reaching 200 ft agl. Below that point, the change of pitch attitude to maintain the glide speed, followed by the opposite sense change of attitude before the flare will all happen too quickly for the student to manage safely.