For actual elements you should not lose anything. Unless you lose something into the air in some smoke etc. you're not going to lose any potassium as heating won't destroy an element. But the compounds those elements are in will change, obviously anything with water will be broken down.

If you just want to avoid introducing or reducing the amount of fruit flies try freezing and burying. Freeze things for a couple of days, then when you feed (thaw first) cover it with a layer of old VC (or bury it under a small layer of VC, just don't go really deep). I find the worms quickly populate the layer of VC on top of a feeding. Also feed in smaller amounts so it doesn't hang around so long uneaten by the worms. Having a tight lid or mesh helps so no flies can get in also helps.

Not sure if you have an bin to recycle or make VC (or both), but just remember you're spending a lot of energy cooking your feedings.

I'm a big fan of freezing, too. It makes the stuff break down faster in the worm bin. And since the freezer is freezing anyhow, it doesn't use any additional energy. I don't thaw however. I just prepare a small hole, throw some dry bedding in the bottom of it, to catch the moisture when it thaws, then put the frozen foodscraps in and cover with VC.

I've placed frozen pumpkin directly on top of vc in my stacks with no adverse effects (that I could tell anyway) and the pumpkin was devoured quickly.

I wouldn't bake stuff, that sounds like a huge waste of electricity. I worked for a company called The Worm Factory, and the owner, Gerrie, always told me to keep 4 to 5 inches of shredded newspaper on top of the compost. We also used to have a towel loosely draped over the bins. We had a problem with some flying bug, not a fruit fly, maybe a fungus gnat, and so I had the bin filled up as much as I could with shredded newspaper. This made it hard for them to come out of the bin and breed, and also to get back in and lay eggs, and they slowly tapered off and now we rarely see them flying around. Maybe if you want to discourage fruit flies, try not to feed your worms anything sweet for a while as well.

On the note of the nutritional value, I don't know which nutrients specifically would be destroyed, but I know many are by high temperatures.

I think drying and powdering is one of the best ways to prepare food for worms. It allows for very precise moisture control, makes it easy to distribute the food evenly over the bedding, and makes the bin contents look less like garbage and more like dirt.

I wouldn't worry about the lost nutrients. Essential (for worms) amino acids and vitamins that are denatured by heat can probably be synthesized by microbes as long as the elements necessary for their synthesis are still present. So I would just try to avoid burning the food.

Can you imagine day in and day out baking food scrap for the worm bin? You will be sick of it really soon, not sustainable long term and waste of energy and time. Freezing does not help because after defrosting, fruit flies will come. Covering the food scrap in the bin with shredded paper or shredded cardboard is the easiest way if you have a crosscut shredder. What's what I do, killing 2 birds with one stone(recycling paper and cardboard). Completely covering the bin with gardening insect barrier fabric is another way. I thought of building a cover with window screen, haven't gotten to it. It will be part of a new setup with leachate flowing down to a bucket from a catch tray, cover whole setup by screen. Can you tell I am just dreaming on...

Regarding speeding up decomposition...what's the rush? Let the worm bin work for you, not the other way around.