Your #define is_user_valid_alive(%1) doesn't do anything special, it just checks if the user is a player, alive and connected.
1. You can't be alive if you're not connected.
2. It doesn't really matter because you then go on and loop through all the players.
A better solution is to use
http://www.amxmodx.org/api/amxmodx/get_players
You should check out the other semiclip plugins as yours might be glitchy with that 0.1 sec check. There is also a semiclip module or two around.