Learn English – Inversion after “only now”

Passive does not invert its auxiliary be; it just inserts it and turns the verb to past participle.
Promoting the direct object to subject is not inversion, which is simple re-ordering of words.

The "must be turned into" part of your proposed transformation is wrong, with preposed only.
Inversion is not required with preposed adverbs, although it is optionally grammatical
with preposed negative adverbs of place, time, or circumstance (i.e, ones that negate the clause).

• At no time was the temperature changed.
• Nowhere may the temperature be changed from the thermostat settings.
• Under no circumstances is the temperature to be changed.

Only, however, is not one of those. If you try this with different adverbs, you get ungrammaticality:

• *With no instrument was the salami sliced by George.
• *In no particular way did she say the sentence.
• *Only is the temperature changed in the daytime.

Your answer is not only perfectly acceptable, but a more accurate rendering than the one they want.

In their answer they have tacitly applied an implication (which is not even a logical implication, but a material one) that 'X is difficult' => 'X occurs rarely'. I think this is inappropriate in a grammar book.