Kellymom has some good information on teeth scraping @LumpySpaceCow if that helps.
If he’s taking some formula from a cup, that’s good. Has yiu probably know, BF babies only take around a floz an hour, around 25floz in 24 hours so they will rarely take more than a couple of ounces when you offer formula at first, don’t let that put you off though if you are returning to work 
I think it’s also normal to feel overwhelmed, especially if you have sleep deprivation. I’d suggest talking to a BFC on one of the Helplines about the indentations and your feelings, they will probably help you through and should also be able to help you come up with a back to work plan.
One thing that really helped with our DS was getting the naps sorted. Have you read the No Cry Nap Solution?.
Another thing that might help is to get DH to sleep with the cot sheets for a couple of night then put them in the cot. Have you tried swaddling and white noise too.
I went back to work when mine werearound the same age but I did do shorter days. On the days I was working I fed first thing, on pick up and before bed (and most of the sodding night) and on the days I was home, they fed when they liked. The days I was at work they had full fat cow’s Milk and solids.
I’m just wondering too how much he eats during the day? I’m not a massive fan of trying to overstuff babies but the Caroline Walker Trust Guide has some sample menus for this age that you may find useful. The only thing I’d probably swap with the baby rice suggestion. We had a bit of an issue with our teen eating loads of junk, I read the CWT guide for teens, changed slightly what we were offering and the carb craving has been curbed a lot. I’m just wondering if trying to roughly follow the guide might help your LO with naps? The guide is quite long but if you are massively struggling to read it, you can always scroll down to the 7 to 9 month section
31 ways to get your baby to sleep and stay asleep might help too 