You can. If you have tons of patience and spare time. lol
If you put -tools in the game's launch option, launch game, click Tools in the top menu and you will see they have Material and Particle editors. You can't open the main game's VPK things in game, so you have to extract them with GCFScape, copy them to Desktop or wherever you want then you can mess about. But I have no clue how to edit one let alone hundreds.

Yep, that first bit is right. It works sometimes. It's not very reliable because it worked for me on a trade server. But after hopping to a Skial server (that allows mods) it stopped what I wanted.

Nope, but I found a temporary idea. Making a custom particles_manifest.txt.

This is what it looks like: http://pastebin.com/Thf8K2NX and it's from tf2_misc_dir.vpk > particles.
It goes in \tf\custom\my_custom_stuff\particles.

You just delete the ones you don't need. For example, mine is:

particles_manifest
{
		"file"		"!particles/burningplayer.pcf"
		"file"		"!particles/muzzle_flash.pcf"
		"file"		"!particles/drg_cowmangler.pcf"
		"file"		"!particles/drg_bison.pcf"
		"file"		"!particles/invasion_ray_gun_fx.pcf"
}

One issue though, item_fx.pcf shares a lot of effects with healing effects. Works fine for me, but you don't get to use the console because it's flooded.

item_fx.pcf; halloween.pcf; killstreak.pcf; weapon_unusuals; invasion_unusuals.pcf and halloween2015_unusuals.pcf should be the ones we don't want.