How many completions and what level thereof is required for the Dino Duel/Dino Fighting minigame where you play as the animals? All the online guides say just buy all the humans and tank for Dino Coliseum but I've done that and nothing's changed. No additional game mode in extra crisis. Anyone else experiencing this issue?

It's missing because you didn't bought any dinosaurs yet. In Dino Duel you can only use the dinos, but you can't use them if you didn't bought a single one so far. You can only use the characters you bought.

They're not even available to buy, I only saw the humans and tank in the menu and then I cannot return to the shop screen since unlocking those characters

You have to finish the game again to be able to buy something in the shop. If you finish it again, and buy the remaining characters, than the 5 dinosaurs will be available for purchase. You have to beat the game each time in order to buy the remaining characters. If you beat the game on Hard, you can also buy the Triceratops and the Compognatus after you bought all the previous ones.