Where does the Gaim x Drive movie set between Drive? Trying to catch up with Kamen Rider Drive, so I was asking.

Its set between Episode 9 and 10. Since in Episode 10 they make references to Lupin, one of the enemies in that movie.

Was there any real reason why all new Den-O forms post-series didn't have the original belt's voice? Skull form, Strike form, Hijack form, etc.

I personally thought it was to simply stand out against the normal one.
Really, the only one to make some sense is Strike, since isn't he from the future? I don't know why other Den-O belts don't have the same voice.

Where was the part in the last episode of drive with chase around? What episode was it during that time?

I thought personally, that the Strike form belt had Ryotaro's voice, alluding to him helping develope the future Den-O stuff. But... yeah, doesn't make sense for the skull/hijack belts.

It never showed that part in the show, but it was before the events of Banno and after Kamen Rider Chaser.They just wanted to create a back story.

Heart (Super Evolution State) vs. Overlord Baron. Who wins?

Tricky. Like with Drive, if the enemy can't get past the Slowdown it might be over before it even begins.

But: Toei always intended for the Overlord-powers to be "out of this world"; with the Taisen movie they even wanted to implement that Kouta as a human Overlord is immune to time alteration. They didn’t because they had to waste too much time with making this movie awful, but I still think the idea is cool.

So if Baron is immune to Slowdown he would win. He's much more mobile, I don't think Heart's golden wave of happiness could get him in his aerially cloud form and Baron can control plants and summon nearly infinite amounts of Inves.

I think they are about even on the power level but Baron's arsenal is bigger.

Also it’s Kaito, so scrapping a shiny golden Roidmude is just another display of his power.

Did they ever make a Den-O toy where the armour unfolded for the different forms like the show? (Even though it'd probably look awful)