You can get both items together in one claim. Hopefully the delay compensation can be dealt with easily and quickly.

For the downgrade, there is a worked example via the wiki. The downgrade proportion relates to the sector downgraded, so JFK-LHR, over the whole itinerary (presumably a two way ticket?) You can try for the companion voucher rated up to the equivalent Avios but the usual view is that you still got the CV to work for you, just in Club. Do the Mennens calculation - at least roughly - so you know whether they have accurately got Mennens in their recalculations.