Caution. *Sein sollen* has some tricky meanings. Same with the other auxiliary and modal combinations.

> Wenn es nötig sein sollte.

is simply another way of saying

> Wenn es nötig wäre.

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1. All the occurences of *wollte* in Kafka's text can be replaced by *würde*. You lose the intentional aspect that way, however. That had to be guessed then. For the first *wollte* it's obvious because of *ja gern*, for the others not so. It would read as if Karl had a lucid dream instead.
2. Same as 1.
3. Not without a drastic change in meaning. If you write *wollten* you imply their intention is not successful. (And it's much more common to write *wollen würden* in that case, because *wollten* could also mean it's past tense.)