i read somewhere replays in lfs is not just some positions stored, but actually a simulation based on recorded user input.
So if i understand this correctly, its a 100% guarantee no cheating was happening if replay is valid and doesnt give any OOS messages, right?
Would it be a bad idea to store kind of multiplayer 'hotlap', parallel offline recording of user input, actual positions and interaction feedback from other players, which then is sent to the server and checked after the race for validity?
Its not a suggestion or request since i understand there are plenty of top-priority things. Just a theoretical fabrications.

Can't guarantee, multiplayer stores only a handful information regarding user's car. They rely on packets sent by others, and not everything is sent. These unsent parameter can then be modified by the user and to which extents give them advantage over other.

i understand in multiplayer only some packets are sent within some time intervals, but on user's machine actual simulation is happening, with opponents car as obstacles. I mean, its always server synchronizing to user, making cars jump and jitter. On a client side client car moves smoothly as if it was hotlap just with abruptly moving obstacles of other player's.
Client can alter parameters whatever he wants, but if client is obliged to store his input during the race, which is then sent to the server, server would have: initial car setup, client's input, moving obstacles (other cars) positions which server itself was sending to the client - there is no unknown parameters, we just re-run the sim on the server - and if there is a difference more then 1-2 meter at the end of the race (or we could shrink it to per-lap check) player either sent wrong setup, changed it during the race, altered positions his client was sending or any other trick. Basically the only way to overcome this system (to generate a valid user input which is in sync with all the hacking) would be to write a driving bot faster then native lfs AI, which seems like pretty non-trivial task to put it mildly.

As I understand it, the replay stores all the sets of all the players and then the positional data is just the same data as you'd get in an MCI insim packet, plus steering/throttle/brake/gear/etc as in a CON packet.

LFS then uses the setup and input data to simulate the driving between these positions from the packets (as it does for other cars in multiplayer), so that the cars drive smoothly instead of being a 5 fps stop-motion kind of thing.