I have found that cheats have to be enabled at the start of each mission. Once the cutscene is done and you have battlefield control, disable the Unlimited Health option, hit Disable All Mods (even though I am not using any mods I still have to do this) then enable Unlimited Health. One way to tell for sure that it’s working is to run an infantry dude through a Tiberium field - if he doesn’t die, you’re good to go. However, your infantry can still be killed if an enemy runs them over with a tank.
I have not tried One Hit Kills - I don’t trust these. Most of the time a cheat offers one hit kills, it also applies to the computer AI.