1) Quartz vs Ceramic Chemistries:
I am aware of three major MH chemistries, those being sodium-scandium, sodium-thallium-indium, and dysprosium-thallium-cesium. As far as I am aware these are QMH chemistries. Are different chemistries used for CMH lamps, and if so what are they? I've heard of the early tin-sodium lamps but I am thinking more mainstream.

2) Pros and Cons:
So there's a few different chemistries, what are their pros and cons? I see some MH lamps with vacuum outers and some with nitrogen outers, when is one preferable over the other? Also some MH lamps lack reflective end coatings, when is this necessary?

3) Probe Disconnection:
Most probe-start lamps disable their probes with a bimetallic switch. Typically this is done by shoring the probe to the electrode near it, keeping the resistor in circuit. However I have heard of lamps that actually isolate the probe. Why wouldn't everyone do that? It's wasteful to keep the resistor in circuit.

4) Probe Boosting Circuitry:
According to this lamptech page, GTE Sylvania developed a metal halide lamp which used two resistors and a zener diode to increase the voltage available at the probe. I haven't the slightest idea how this would be at all possible without the use of a capacitor, but I said the same thing for FEC ignitors and I understand them now lol. How does this arrangement work?

Thanks!

Ugh! Your questions worth a rather thick book on MH technique )

Some quick thoughts:

Alumina arc tubes in ceramic MH lamps are less reactive to the filling salts and allow for higher temperatures and pressures. So classic fills like Dy + Tl + Na just work better (due to spectral line widening) but also CMH allows for some unique new metals. Probably the most significant is calcium, which under CMH conditions radiates a few  wide bands in deep red, allowing for creation of CRI 90+ 3000K lamps, finally with a good R9 rating, which usually suffered in other MH lamps, due to inevitable emphasis of Na radiation on orange.

Probe disconnect by a normally closed switch is just mechanically more cumbersome than a simple normally open U-shaped bimetallic strip placed between cathode and probe leads. Look here https://www.lighting-gallery.net/gallery/displayimage.php?pos=-229314 and here https://www.lighting-gallery.net/gallery/displayimage.php?pos=-229361

Probe ballast resistor is a relatively high value, somewhere around of 10 kiloohms, so actual power losses are not especially significant.
Anyway, probe start lamps were a long time obsolete tech many years ago already. All high performance designs are pulse start.

Regarding a zener and voltage increase, I have no info on this design. May be a patent dig will surface some info. Sure, you can't much increase OCV with just diodes/resistors, but there may be an old trick applicable for CWA circuits only. Adding some kind of diode across the lamp will cause half-wave rectification and charge CWA circuit's output capacitor. That way, lamp voltage will become asymmetrical, with some boost applied every half period.

Also, for CWA circuit, asymmetrical discharge between probe and nearby electrode may charge CWA capacitor in an undesirable way, lowering OCV peak when nearby electrode is a cathode and far electrode is an anode. Adding extra diode can counteract this.

 

Regarding reflecting coatings and vacuum-nitrogen fills.

Every arctube has its optimum design temperature inter-related to working pressure. This depends on the projected lifetime and also on getting optimal efficiency, also a color balance needed.

Usually, all white lamps have these coatings, with a notable exception of late generation of 400W Philips HPI-T lamps. I can bet HPI-T lamps have smallish highly loaded arctubes which can easily reach the temperature required without extra measures, and do not suffer much from cold spots at the ends of arctubes. Also, tri-band fill used does not call for high vapor density, risking to lose some efficiency by turning narrowband emission into wideband and spilling energy into IR/UV.

Colored MH lamps with Ga or In especially usually skip reflective coatings, as their arctubes need to run relatively cool, at low metal halide vapor density, else they turn their desired narrowband emission into inefficient cold white color.

Also vacuum or nitrogen is chosen depending on the desired cooling of the arctube. Older larger arctubes with lower loading called for a hard vacuum to be kept hot enough. Newer smaller/shorter arctubes may need nitrogen for extra heat dissipation. Also, different fills have different optimal temperatures. So lamps with NaSc or tri-band chemistries may be nitrogen filled, and rare earth ones need to run hotter  and may be hard vacuum.

Scandium can't be used in CMH lamps because scandium is very corrosive to the ceramic, compared to quartz.