Sentence Analysis--except--conjunction or preposition?

a) The second conditional expresses a present or future tense except when the conditional is embedded within a main clause. In such cases, the tense expressed is that expressed by the verb in the main clause.





1) Is except a preposition and thus the words following a noun clause?



2) Or is it a conjunction?





3) Also, are both these sentences correct alternatives to the above?





b) The second conditional expresses a present or future tense except when the conditional is embedded within a main clause, in which case the tense expressed is that expressed by the verb in the main clause.



c) The second conditional expresses a present or future tense except when the conditional is embedded within a main clause, where the tense expressed is that expressed by the verb in the main clause.





4) What are better or alternative linking words to 'in which case' and 'when'?



5) Do you think the pronoun 'that' clearly enough refers to 'the tense'?





Thanks