Update:
It is difficult to say much, and this is already very close to spoiler territory. But I think the purples are about right, and not too light.
After some experimentation, I think I understand what Alex was concerned about.
A purple landing from above does not smash through the Batman structure very easily, and often stops above a wing, or bounces away to one side.
If it does manage to penetrate, however, it then seems impossible to stop in the lower structure. It changes from feeling too light, to feeling much too heavy.
The plates and anchors protecting the portal make it very hard to get there. Which is exactly as it should be.
The difficulty of using the purple dillos to make a path to the portal is intense, but can be done. I have managed to get to the portal with not that much building, but a lot of tuning.
Alex need not worry.
I think the balance is almost perfect. Just when you think you've found the Goldilocks solution, it finds a whole new way to fail!
And, I can see from my experimentation, that there are a
lot of different ways to solve this.
In my opinion, this is an
amazing challenge.
EDIT: PS I haven't actually achieved a finish yet, but am responding to the concerns expressed.
And, I think it will be so difficult to get a purple to the portal, that it does not matter which one of the three is used to complete the challenge.