The ones that are not portraits of players have been in the game files for quite some time - they are not new. So I don't think we can hope for any content being released in the near future.
You can just add the health_maximum modifier to the upgrade itself. Since that modifier will also heal the unit up to its new maximum health, you should probably only add the upgrade after a delay. You might also want it to be automatically removed once the extra health is taken away by damage, and that could be a bit tricky.
For player abilities like call-ins or artillery, removing them from the army's ability list will usually get rid of them. Squad weapons and upgrades can obviously just be removed from the squad. For anything else, I guess you could just remove all actions and make the button invisible. But there's no quick way to disable everything at once.
Have you already tried to create the Sherman Crocodile? I tried it for my own mod, but I couldn't get the flame animation working.
I tried making an OT-34 flamethrower tank and had the same problem. With the method you used for the Hetzer's gun, I could get some flames to appear, but it didn't look good. It might be possible to make something with the FX editor, but I haven't used that before. What about you?