I'm quite certain TI won't change the emulation. If they do, it will be, well, worse for us, as you said. But I doubt it will change. It will probably just be more inefficient if it does.

That's not something you would ever want to use unless you're seriously abusing the consequences of its normal use, which I only did once when I tried grayscale double buffering.  The sp register is the location of the stack.  If you change it, you will no longer be able to call or return from subroutines, push or pop variables, and some other nasty things.  Not to mention there's a lot of roundabout code you would have to use to even load stuff to or from it.

It is bad 

For debugging purposes there are times I wished the Z80 had better methods of accessing the stack pointer

If you run out of room, you just use some free ram location.  You have to save the current stack pointer somewhere anyway before you can use it, otherwise the calc will crash becasue it can never return back to the operating system.  If you're not taking advantage of the automated push/pop features that are inherent to the stack pointer, then just use a ram location becasue its smaller and faster in ever way.  Trust me, you'll NEVER use the sp register until you get into the really advanced tricks, and even then you're not using it to store numbers, you're using it as a fast way to read or write to ram.

I don't think being able to access the high/low bytes of SP directly is even possible. If you are going to be using SP like some "normal" register, you'd have to keep a few things in mind, some of which was already stated in this thread:
1. You need to save SP somewhere since it's usually important for the stability of the system. Use instructions like "LD (xxxx),SP" (where xxxx is an address) to save SP, and "LD SP,(xxxx)" to recall it again.
2. Interrupts need to be disabled while you're messing with SP. The instruction "DI" is great for that. Interrupts randomly make CALLs. See point # 4 as to why this is bad.
3. There are a few other instructions you can use to mess with SP or deal with it. That's where it starts getting cumbersome. Just let me know if you want to know, either in this thread or via PM.
4. The stack is no longer available (unless you're creating your own miniature stack for some reason). This means that you cannot use PUSH/POP without destroying whatever it is you're working on. Same goes for CALL/RET.

Now... with respect to IXh/IXl ; IYh/IYl. A simple way to remember where you can use them is... wherever using H or L is a valid option, you can use the combination above. Except when the instruction calls for the use of HL or IX/IY as well. As stated in an earlier part of the thread, none of these combinations will work on the Nspire since these uses are "unsupported" instructions. Also, the OS uses IY for its own purposes and expects that register to be the same when the program exits. So be sure to save IY as well. Or if you happen to know what's loaded to IY (it is some constant), load that value instead (saves time).

And as stated (somewhere) above, only use these combinations if you absolutely know what you're doing. Their use is more like a last resort than anything else.

Yeah, when I said you never see sph and spl, I meant that literally. You cannot access the individual bytes.

*DRUM ROLL*

Lesson 9 is up in all three formats.  Also, lesson 8 .html is up.