Proč nemůžu říct: Kateřina takes František For a good man.
No-one has reported this yet in our system (not even you).
"to take someone for someone" is mostly used in idioms and fixed expressions (e.g. take for granted, for an idiot). Let's hear the opinion of native English speakers.
While a phrase like "What do you take me for" is commonly used, and might be answered with (as VladaFu suggested) "I take you for an IDIOT!", I (native AmE) really wouldn't expect to hear "take X for" used outside of that context.
Why is “za” reqired here? I don’t understand how it fits into the translation. Is it always required with the verb “považuvat”?
yes (there are some archaic eceptions)
Does "za" go with "povazit"?
Would "a good husband" be acceptable here?
Yes, it's also accepted - it's possible. I would personally not interpret it that way (I would expect "manžel" here), but it can mean both in the right context.