I'm not in a PhD program, but several close friends/family I have are in them now. All of them have had their relationship with faculty in their program, particularly their PIs/lab leaders, make or break their experience. If you don't feel completely welcomed, accepted, and supported by the PhD program that admitted you, I wouldn't go. It's a long and difficult process in the best of circumstances, and going somewhere that you already know doesn't provide the support you want (and isn't fully funded) doesn't sound like the best choice. You seem like a strong candidate now, and I'm sure if you apply for PhD programs after your MPH, you'd have several options to choose from that are stronger than the one you have now.