MCCC Help!!! How to stop it messing with my families until I'm ready.

Depending on what you want open up mcc and on the main menu click MC population

i pulled the following from mccc documentation. these won't solve all your problems, but they'll help with some.

MCCC > Flag Active Sims... > MC Pregnancy > Flags Affecting Marriage/Relationships > Flag Never Marry (no random marriages, applies to all active sims in household)
MCCC > Flag Active Sims... > MC Pregnancy > Flags Affecting Marriage/Relationships > Flag No Romance Move-Ins (no random romance move-ins, applies to all active sims in household)
MCCC > Flag Active Sims... > MC Pregnancy > Flags Affecting Sim Pregnancy > Flag for No Affairs (no random affairs, applies to all active sims in household)

in order to prevent random death, i would just flag all sims i don't want to die as immortal, then unflag them when i'm ready to let them die. maybe someone else has a better solution, though.

edit: oh, i was clicking around & found some other settings. these you have to access by clicking on the computer

mccc > mc pregnancy > marriage sim selection > bypass played households (when checking for random marriages, bypass any played households rather than just the current active household)
mccc > mc pregnancy > marriage sim selection > bypass active sim romances (during random marriages bypass sims that are in a significant-other relationship with an active sim)
mccc > mc pregnancy > pregnant sim selection > bypass played households (when checking for random pregnancies, bypass any played households rather than just the active household)
mccc > mc career > bypass played households (when assigning careers to Sims, if this is true then any household marked 'played' will be bypassed by the sims 4 career-assignment process)

I am using mccc for years without the story progression and it works fine. I just went through all the options in the population and the pregnancy module and switched it all off. Don't forget you get different setting when you click on a computer, a sim or a mailbox. All 3 offer some part of mccc. I only did this when I first got the mod so I don't remember the options I choose but I know it is possible. I didn't have to flag individual sims as immortal just change the general settings.

You could simply remove the Pregnancy and Population modules (and possibly also the Occult module) if you don't want ant sort of story progression.