my concern is that perhaps she should consider changing her major (she has to do so before entering the university and get approval from the school) to a 'lesser' science [my perception of something less taxing] other than Chemical Engineering.
Your questions and concerns are spot on.
This was my DS's experience. He commissioned into the Army in May 2016 so his situation certainly is not analogous, given the different services and the changing needs of those services. He earned a 4yr scholarship AROTC and attended a Big 10 University with a very strong and well known Chem E department. In High School he was off the charts in Math and Science. He loved Chemistry the most. He also saw the Forbes and WSJ rankings of the highest paid grads. Chem E was right near the top (crude oil was over $100/barrel at the time), so that sealed it for him. He also had the advantage of carrying in three semesters of Calc and scored a 5 on AP Chem, so he arrived with a semester under his belt. No cadet/mid was ever better prepared to handle the rigors, logistically and intellectually, of a Chem E major and ROTC. Well...
What does an 18-19 year old boy know? Not as much as they think. Everything was great and easy first year. Second year, when he got into the meat of Chem E, he realized that he hated it. I remember, "Dad, it's boring. I have a 2000 cell spreadsheet and I have to make the upper left agree with the lower right. I can write a proof of almost anything. I can problem solve. But, they serve no purpose in these classes I have to take." I'm not excusing him from anything. He could have done the work, but he didn't want to. He also figured out that even with so many credits carried in, he had virtually no free electives. He wanted more CS courses and wanted to study Arabic. The Chem E major had no room for those within a four year time frame and there was no way on God's green earth he was going to stay in school an extra year, even if the Army would pay for it. He was an excellent Violinist and hasn't played since he brought the instrument home first Christmas. He loved tennis, but never really had time to play until his junior year.
His solution was to change his major to Chemistry. He took courses, inside and outside of Chemistry, that were more interesting to him. He got a job in a lab actually "doing" Chemistry. He had a high GPA and AFT, so there was no pushback from the Brigade about switching his major. The other thing that hit him around Junior year was the realization "Hey, I'm going to be in the Army in two years. If I'm going to be in the Army, I'm going Active Duty and I definitely don't want Chemical Corps." He ended up going AD, branched Signal Corps (nothing to to with Chemistry) and is a happy camper.
Long story short. If your DD knows what Chem E is, likes it, has the work ethic, and is prepared to make all the sacrifices, then go for it. She wouldn't be the first xROTC mid/cadet to make it work. I know my DS worried more about the perception of failure than whether the Brigade would let him change majors. Only your DD can figure out what is and isn't important to her. And that can't be done ahead of time. She will learn quickly the meaning of the quote, "Success is not final, failure is not fatal: it is the courage to continue that counts."
Best of Luck to your DD!