A book for learning Italian has written "ora tocca a lei". Shouldn't it be "ora le/loro tocca a lei"?

No. According to the traditional rule one has to use just one pronoun with the same reference. Either "a lei" or "le". Other example: "a me piace", not "a me mi piace". Spoken language is less rigorous.
In Spanish is different: the rule is "a mi me gusta", "a mi gusta" would be a mistake.
No is the right form is the rethorical way to say: is your turn now
No, what you're talking about is completely different from the first sentence. "Ora tocca a lei" might have the following meanings:
1) it's now up to her;
2) it's now her turn (do do something)
3) it's now YOUR turn (courtesy form!)

the sentence you suggested would mean "now they touch her"; and the correct form would be " ora loro la toccano".
Hope it helped, and hold on! Our grammar is extremely complicated, so don't give up!


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