Huge hugs to you,
First of all - what is your plan now that you're back in work?
Second - don't worry about the open cups saying 9mths (they have to give an age and are maybe worried about babies being left unsupervised with these cups). Even premature babies are tried in neonatal units on open style cups (like little medicine dispensers) with colostrum inside so there is no reason to suggest that your DD couldn't go to an open cup - and it would be much better for her at this stage anyhow if you plan to bf till a year or more...
thirdly - let me give you my story. I went back to work with EBF kids at 4.5 and 7.5 mths. With DS, my first, we had no issues switching from breast to bottle and back again, even mid feed. He was fine. Fed him bm direct or from the bottle till 16 mths - no issues. We did, though, drop that 11am feed before the DF. I wanted the skin to skin, wanted to actually feed him to keep up my supply and didn't believe in EBF babies going 12 hrs at night (and needed the sleep being back in a demanding job). So I fed him 7am (worked 7.30 to 3pm), then he had brekkie, he had lunch and a sippy of water around 11.30, bm at 3.30pm, solids at 5ish and then bf to bed around 7pm, lifted once more for DF at 11pm.
With DD, as I knew dropping the 11am bm in a bottle really helped DS develop an appetite for solids, I dropped the 11am feed as well just before going back to work which meant in essence that I didn't have to pump and with new work schedule, I didn't even have breaks so would feed her 6.30am and work 7-3 then home for 3.30pm. Again, we kept the DF till well beyond 10mths, the 3.30pm feed till a year and then two bfs till she self weaned at 18mths.
I know your schedule is probably a lot worse than mine - my early start (which I hate so much but worked out very well for bfing) meant I could get back for that 3.30pm feed. Perhaps you could try the open cup - even a normal open cup? My DD did very well on typical kids open beakers from a year old (copying DS) and never took a bottle. She did well with the straw as well - but only for water.
HTH and hugs xxx