I don't fully comprehend what you exactly mean..?
If you don't want to wear the Halo, wear a regular diamond helmet instead, and if so, download this pack here https://www.planetminecraft.com/texture_pack/elytra-reality-0002 which only has the wings, no halo.

If you mean if you can remodel a carved pumpkin so it looks like a Vanilla 16px diamond helmet:
That should be doable, sure, you can see a few highres-helmets and crowns in this showcase post here: https://www.planetminecraft.com/texture-pack/model-reality

I can't promise you thought it would look 100% like a genuine Vanilla diamond helmet, given the way that armour in MC is rendered on humanoids.

That's something you'd have to test.

I don't know why you would do that though?

Pumpkins got no direct damage protection.

If you don't explain what you mean by "use it for your server", then the short answer is: No.

You are not allowed to modify the pack and give it to others.

You, for yourself alone, can do with it whatever you like, if it matches the look you go for better (e.g. for videos), as long as you don't share the pack, modified or not.

Hello, with first photo you mean with the golden-yellow wings? That was a SEUS shader, but long ago, I don't know if the current SEUS shader will make the wings look the same. The next picture with white wings, but beautiful clouds, is no shader at all, just Optifine shader feature activated with no shaderpack. The clouds are from a Cutsom SkyPack I use :)