Just as background info; the throttle pedal is entirely electric, so there are none of the throttle cables fitted to your car that were typical on other cars before, say, 2006. The fact that the pedal was physically stuck down lends this to being a purely a mechanical problem (with just the pedal itself) rather than anything at all beyond it, as you probably already knew.
Because the pedal has just a simple electrical plug and 2 or 3 bolts holding it in, removing it to be very closely inspected for grit, stones etc. getting into the mechanism should be quite a quick job. The pedal could be checked for stickiness 'on the bench', as there is no need for it to be back in the car to check that the pedal returns correctly. I have not come across this this problem.
With regards the juddering, starting with the cheapest to fix, I would be thinking about possible ignition coil failing. The Mk1 Auris hybrids have sometimes suffered rainwater leaks into the spark plug area underneath the ignition coils. But if that was the case, I would have expected your garage to have noted seeing rust or corrosion when the plugs were changed. But an error code is usually logged with this fault. . A new coil is the likely fix here. But without a code there it is just guesswork as to which one, or if it is this problem at all.
Or, another possibility given the miles covered - an EGR cooler blockage. But this usually logs an error code of some description as well, and your mileage is really a little low for this. The fix here is to strip and clean the EGR valve, cooler and inlet manifold as per this video:-
There are no parts needed for this, but it is a time consuming job.
How is the fuel consumption? Does the car burn much oil? Any strange noises when the car is juddering? Do you know what plugs your (non - Toyota?) garage fitted, i.e. is it on your invoice? Strange though it sounds, the cars run best on Denso brand plugs, but not to the extent of suffering a judder like yours.
I have nothing against Redex injector cleaner, but as it is so cheap and easy to do, I would try a different brand of injector cleaner, perhaps say, Wynns or Forte, in the meantime.
The above problems are not specific to hybrid cars, this is just normal garage spanner work, the same as checking the coils and changing the plugs.
I'm sure someone will be along soon with some other suggestions........