It's certainly preferable to give an antibiotic when replacing a prolapse - get her some as soon as you can.
I prefer the spoon and I'll tell you why.
When correctly inserted, the spoon seems to be extremely comfortable and stop her wanting to evert herself. Stitches, pins, harnesses and other solutions only prevent her being
able to evert herself - she may continue to feel uncomfortable and even continue to strain once they are in place.
Yes, I'd assume that I would prefer to not have a hard plastic tongue inserted up my jaxie too, but you can see how much more comfortable they are once it's in place.
Personally I would absolutely not reduce her hard feed, if anything the reverse.
Firstly, it can be dangerous to suddenly change the nutritional levels of a heavily pregnant animal, particularly downwards.
Secondly, the prolapse is caused by there not being enough room inside for the food intake, her innards and the lambs. Nutrition in hard food form takes up considerably less room than the same nutritional value in forage.

If anything, split her cake ration in two and feed it to her in two feeds - but don't reduce the overall level.