"She needs to leave now."
I'm not sure I have ever seen 必须 used in conjunction with 要. Why is 她现在必须离开 not acceptable?
必须 and 必须要 are almost always interchangeable. You can interpret 必须要 as "must need", it's more like an accentuation to 必须 (must).
Thanks for the explanation. I've been a Chinese speaker for over a decade and even studied the language formally at a Chinese university for two years (I'm simply doing Duo as a way of keeping my skills sharp now that I'm back in the US), and it just seems a little odd that I've never encountered 必须要 before.
Knowing that you know the language will make the explanation easier.
Think about this sentence: 人必须要有耐心，特别是要有信心。
Without 要 it would sound weird, so 要 is not always dispensable imo, and it's pretty natural here to use 必须要, however, I haven't studied grammar systematically (XD kind of sad to say that) to point out the grammatical reason behind. I think 要 appears because we say 要有，要做，要离开, so it's because it sticks to the verb and not much related to 必须. I would say the sentence is more 她|现在|必须|要离开 than 她|现在|必须要|离开.
Another example: 必须得想个办法。
If we want to add 要 in there, it won't be 必须要得想个办法, but rather 必须得要想个办法, so the 要 stays next to the verb. Adding a sense of emergency/must-be-done undertone to the verb.
Interesting! Your explanation is very simple, clear, and insightful. Thanks so much for taking the time!