So of the 8 different samples, which did I end up choosing? Very simply, #6
. Two coats of Duratex on natural MDF.
(aka "old eagle eyes") correctly pointed out, there was visibly better black saturation with the natural MDF vs. sealed MDF. Here's two photos taken at two different light angles. Top is sealed MDF and bottom is natural MDF.
A clearer indication of the difference - and precisely why I used a piece of painter's tape to keep one area of the sealed MDF as natural unsealed MDF - is reposted here:
2. LEFT SIDE: Sanding sealer / Duratex / Duratex
TAPE LINE: Duratex / Duratex
RIGHT SIDE: Sanding sealer / Duratex / Sand 110 / Duratex
Sanding was quick and easy, but difficult to only knock down the stipple and not sand the base layer. If you sanded through even slightly, this showed through very slightly after the second layer was applied. Impossible to photograph but caused a very slight variance you could pick up by eye.
The water dilution....was interesting. Color saturation was the same as two layers of pure Duratex. The primary difference was the 'stipple' of the diluted Duratex was more flat and rounded. This is because the foam roller created tiny stipple 'bubbles' which didn't last long but created a totally different type of look to the finished surface. It's hard to describe and maybe I'll try to take a photo of it with my DSLR to show what I'm seeing on the sample more clearly via pictures.
The last bit of advice I'd have is don't skimp on the Duratex but also don't go overboard. There's a relatively thin line between sufficient coverage and too much material. Too little and it can look splotchy with less color saturation. Too much and you'll have a harder time smoothing out the roller lines and getting a consistent finish. I found if I applied a bit too much I'd roll out my roller onto a scrap piece of MDF, essentially 'drying out' the roller of paint. When I went back to the sub box the roller soaked up the 'extra' and could easily smooth out the rest for an even finish.