How to fix re-download problem of an undamaged game on Steam?

Well this is not really a fix, but next time you should make a backup of the folder if you're planning on changing game files. I always do that before modding a game, just incase it messes up.

Are you sure its downloading the whole game? RO2/RS multiplayer got an 8.5 GB update today or yesterday

My fix in case Steam ceases to see the game I really have:
1) find the game in your steamapps/ folder;
2) rename it (e.g. add one random symbol at the end);
3) delete the game via Steam interface;
4) rename the folder back;
5) reinstall the game via Steam interface.
Usually everything goes smooth, but sometimes I still have to redownload the whole game (or its corrupted part).