Some very important questions, and no simple answer.
First you state "This brings me back to my housekeeper, who wants IMSS, which means I become her employer. "
If she works for you and you pay her you are her employer. Its called part time.
Question. When you hire someone (maid,gardner) are you required by law to register them with the tax department (includes IMSS)
How many hours a week demands them to be registered with tax department?
What are your requirments regards sick leave, vacation pay, comp when firing when with IMSS.
What is required as above if not with IMSS.
I ran business in both Canada and here in Mexico and will state the laws and tax system are totally differant. I have live full time in Mexico 21 years now.
I have had to deal with letting people go, and also firing people.
This much I will tell you. When I needed to fire my maid for stealing from me (lawyer advised getting the crime on tape, which I did) I learned LEGALLY she was entitled to full comp and that is sad. The only way around getting out of comp was going to the proscuters office and filing a report with the video tape.
This much I will tell you, Mexican law protects the employee and not the employyer.
Also if you do not have a contract you lose. If you do have a contract there are things you must do to fire someone to avoid paying out.
My personal feeling regards having a maid or gardner (which I have) is if they need time off for being sick then get a doctors note and they get paid.
If they are on IMSS that is already built in, they get a note and I must pay them. For extended sick time off there are laws.
If you employ someone either on IMSS or not and you need to let them go know they are INTITLED by law for payment.
Suggest to find out what the law is. I am not qualifed to tell you and will consult my lawyer or employment office in Guad if I need to let my maid or gardner go as I have not had to let onego in over 10 years now.