Halliday & Matthiessen (2014: 474-5):
Finite clauses with whereas, while, except that, if they follow the primary clause, have a strongly paratactic flavour. The line between parataxis and hypotaxis is not very sharp; as a working rule, if the extending clause could precede (thereby becoming thematic in the clause complex), the relationship is hypotactic (since +β ^ α is a possible sequence, but +2 ^ 1 is not). An example where the extending clause could not precede is
||| He pretended to know all about it || – whereas in fact he had no idea of what was happening. |||
This would be interpreted as paratactic. In such instances the conjunction is always unaccented.
Not always. In the above example, if fact attracts a tonic signalling contrastive New information, the rhythm is likely to be (with stressed syllables underlined):
/ ^ where/as in /fact /