how to delete and use other peoples user data

If your client is the host of the server (if you open in Singleplayer and open to LAN), then your player data is stored in two different places. One is the one you found, in playerdata/[uuid], and the other is in level.dat/player. You can't just remove this entry from the filesystem, as it's inside a .dat. You'll need NBTExplorer or something similar.

As for your question,

is there any other good way to quickly search ppls invintorys on a single player world

I don't know what you mean by a 'single player world,' as you said 'server' in the question. I'll just assume it's a server.

If you know some unique characteristics of the lost item, you can search for it in the NBT data of connected players. For example, this will make all players that have a diamond in their inventory say "I have a diamond!"

/execute as @a[nbt={Inventory:[{id:"minecraft:diamond"}]}] run say I have a diamond!

If the item has a name, you can narrow down the search. This checks for diamonds named "Diamond of Truth".

/execute as @a[nbt={Inventory:[{id:"minecraft:diamond",tag:{display:{Name:"{"text":"Diamond of Truth"}"}}}]}] run say I have a Diamond of Truth!

Hope this helps!