What happens if you change your mind and decide not to go back to work after going on maternity leave? Can the employer get pay back from you or make you pay for the health insurance for the 12 FMLA weeks?

The law specifically allows an employer to require you to repay any health insurance premiums they may have paid on your behalf while on FMLA. While the law permits this, not all employers avail themselves of the permission. So you may have to, or you may not.

There is no provision in the law permitting an employer to require you to repay any paid leave, or disability benefits. So you don't need to worry about that.