They idea might seem a little strange, but it takes quite a bit of pressure to move them individually. The next step is to join the rails temporarily, turn the chassis the right way up and put some weight on it to find out what weight the springs can take.
As JFK once said, "Just because we cannot see clearly if HDPE springs will work, that is no reason for not setting out on the essential journey"

I totally agree with your sentiments. Suck it and see.
With a steel spring part of it's job is actually locating the axle in fore and aft (making an assumption of fore and aft springs) and also dealing with the rotation of the axle.
This method will need to have ways to deal with that, however there are a multitude of ways to cope.
The load potentially going onto the front end is more likely to be around 100 kg. A deflection rate of around 35 kg for 35mm per spring has been used for other cars successfully.

You could easily put a block of something on the bottom of the chassis
rail, over the center of the springs to stop them deflecting enough to go inside out...just a thought.

Any progress with the HDPE springs can now be followed on my build journal "old number one"