Hi there! There are 2 questions to answer in your situation:
(1) Retail vs Office position - it is true that office-based positions may have slightly more regular hours. However, sometimes, it can come with different types of stress AND you might have to bring work home to do. If you were to be promoted in your Retail position (as promised to you), would you stay? Because as a manager, the hours and workload as a retail employee would be very long (much longer), so would you have stayed, if you were promoted? These are the questions you have to ask yourself first, before making your decision.
(2) New HOD - so far all you have heard are rumours of new people coming in. If you decide in (1) that a promotion would result in you deciding to stay on, then I suggest having a talk with the HOD first to understand what the plans are for you. Take the motherhood out of the question - ask about this promotion. Remind him/ her - "I am a mother, but also an employee of this firm." Starting a new job in a new company may be good - but bear in mind that there will be new learning curves, and there is no guarantee that the new supervisor/ boss is easy to work with. You will also need time to adjust to the situation. In addition, having a baby is a big change. You might want to consider if you want another change (at work) because sometimes too many changes at the same time can be VERY stressful as well. Based on that alone, if you decide to move on to a new place, you might want to consider waiting a few months (Settle into motherhood first) before doing so.
Hope this helps you somewhat. Good luck! And I strongly believe that every mother is doing her best with her own situation. And I am sure you are, as well. Enjoy motherhood! :)