You can use my set heath plugin to force the health amount when any players spawns into the map.
https://forums.alliedmods.net/showthread.php?t=151738
The plugin compiled.
https://www.sourcemod.net/vbcompiler.php?file_id=192029
Ther are two setting and for what you are doing you wil want
abs_l4d2_StartingHealthTimePeriod
set to some high value that would be at least as long as the longest map time like 20 minues or 1200 seconds
// Sets the health of the survivors at map start or at any spawn depending if the time below has expired
abs_l4d2_StartingHealth 100
// Set the time period in seconds after map start
// to set survivors health when you take over a bot
abs_l4d2_StartingHealthTimePeriod 1200