In the US? V3, Kuuga, and Zero-One. Note that V3 is insanely expensive because it’s licensed by a tiny company based out of Hawaii. The other two are on blu-ray, not DVD, if that matters.

Has it ever been explained, either in series or behind the scenes, why Amazon's chest piece changed colors?

I’m assuming you mean Showa Amazon-I’ve looked and found no explanation, except the red colored chest piece is called “Amazon Late” I think it only appears in his series and maybe the Z-X movie. I figured Bandai would put out a SHF of it at some point, since it’s a simple recolor

The red chest is apparently what it was originally supposed to look like but they couldn't get it right until a bit later in the series. Yellow chest is the one that's most known, though, so red has made very little appearances aside from Senki

Why does Stronger have such a different helmet design/construction than every other Showa Rider?

He's built different.

Oh obviously. (I don’t care what Tendou says) I just wonder why they went with the oversized helmet and ventilated not-compound eyes. Was it to try and make the suit actor more comfortable?

Not really Kamen Rider, but it's adjacent. Why isn't Kikaider considered part of the Metal Heroes series?

That's a good question. Best guess I can give is that, because they chose a kabuto beetle motif for the first time, they wanted the suit to reflect their size and armor. So he was given a bigger helmet and a massive chest piece.

Stronger used to be a football player before he was captured and modified so maybe the larger helmet is supposed to evoke a football helmet?

I don't get the "adjacent" part, but there wasn't a Metal Heroes series during the time of Kikaider! There were multiple '70s tokusatsu, just as there were multiple '80s horror/slashers. A veritable glut of material all vying for attention. When a thematically situated property or concept garners mass attention and positive financial returns, it leads to additions and sequels and competitors and people throwing things at the wall to see if anything sticks. Kikaider was one of the ones that stuck.

Once Gavan and the Space Sheriffs made their debut and more or less succeeded, they tacked on anything they could think of to try and copy that initial success.

Kikaider just wasn't retroactively considered to be a Metal Heroes concept like Gorenger and JAKQ were retroactively considered to be Super Sentai concepts. Partly because Kikaider was in the '70s and Metal Heroes can be considered a firm '80s-'90s concept.

I could argue that it's because Kikaider wears a "suit", but Jiraya renders that a virtual lie.

It might seem unfair, but Toei doesn't always double check their work! Should Ninja Captor be a retroactive Super Sentai series? Some have pondered the same.