Add a line promotion

Promotion eligibility can be nuanced to put it lightly. Let me clarify the situation since I cannot see the account. 

There may be more on your account, but the minimum that I can glean from your posts is that you, your spouse, your brother, brother's ex, and now your mother-in-law are on one account. You added MIL's line and got the $700 off the Google Pixel. MIL is getting a discount on the phone but NOT the line. Now you want to remove the ex's phone number (which makes complete sense). There are two types of recurring credits; Device and Line. Since this situation is a Device credit, the requirement is that you keep the device payment plan associated to the phone number you added to get the phone and THAT phone number stay active for the length of the payment plan. In this case, you are good to cancel ex-girlfriend's phone number and not affect MILs promotion. 

IF ex’s phone number has a discount or she got a promotion on her phone, her promotional credit would stop.

Sorry I am a bit late in providing this answer, but I wanted to make sure 100% before I threw an answer out there. Have a wonderful day!