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.

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.)

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.

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.)