Masked Rider/WMG

Dex is an Involuntary Shapeshifter.
This is why sometimes he resembles Kotaro Minami.

The Masked Rider armour changes form sometimes to protect itself.
This is why it sometimes resembles that which was worn by Kamen Rider ZO and Kamen Rider J.

Dregon left Earth to conquer another world by the time of Power Rangers Zeo.
In the episode of Zeo where the Red Battlezord is introduced, Zordon says something along the lines that it is the last line of defence on Earth (now that the Zeo Megazord is out of action), implying that Dex has left Earth to fight Dregon elsewhere.

This also explains his absence during Countdown To Destruction; he was fighting Dregon's forces elsewhere.

The version of events we saw was a Show Within a Show in the Power Rangers Universe
This is how Nadira got to watch the show in Power Rangers Time Force. After defeating Dregon, Dex sold his story to some tv execs.

This also explains the differences of Dex's departure between "A Friend In Need part 3" and "Escape From Edenoi part 1". In the former episode, his grandfather tells him that there is no hope left for Edenoi, implying the planet is about to explode or something. Then in the latter episode, Dex is told by his grandfather to leave Edenoi again, without any sign the planet is going to go bust. It also explains why Doubleface acts as Dregon's second in command in the former, but not in the latter, how was Dex to know the chain of command inside Dregon's ship?

Doubleface got a demotion after "A Friend In Need part 3"
This is why he behaves as Dregon's second in command in that episode and why afterward he has the same role as the others.

The Masked Rider will return for the next Power Rangers series.
It's been confirmed that Samurai Sentai Shinkenger is getting adapted for the next season of Power Rangers. Shinkenger featured a crossover with Kamen Rider Decade. Hopefully, if the writers will be mythos-savy enough to recall the Power Rangers / Masked Rider crossover and take advantage of this footage to have Dex appear in the new series, either as a reboot or a continuation from his arc. And they'll do it right this time. Please?
 * Problem is, Decade's suit looks even less like Black RX's than ZO and J's did. If they use the Decade footage, it would make more sense for the Riders to be new characters. What they should do is adapt Double and then let the Goseiger adaptation cross over with that. A lot.
 * HOWEVER! They may handwave it with that the Decade suit was made to replace the original Masked Rider suit because the original wasn't enough to fight the evil on Earth at the time or something.
 * They could also state that since we've last seen him, Dex has defeated Count Dregon, his father has passed away, and he's now the king of Edenoi, and that the Decade suit is his royal suit of armour. After all, purple is a colour of royalty, and magenta is close enough...

Dex died on his way to Earth after a Freind in Need Part 3
Considering Masked Rider is likely not part of the official Power Rangers universe, and Dex was never heard from again. Instead of fighting Dregon on Earth, he sacrificed himself to destroy him.

The show is something Pilot Dex or his family cooked up to cover up fighting real monsters
I mean it's not like Power Rangers are real or anything

Albee being black despite being in a White/Asian family is a reference to the source material.
The Kamen Rider Black transformation was originally known as "Black Sun". Albee is the Stewart family's black son.