Hehehe...
4 nights in Brussels for conference
2 nights in Luxembourg (one full day)
4 nights in Paris (three full days)
3 nights in Amsterdam (two full days)
Does that sound "right"??? LOL...Hermes in four countries...plus sites...
I think 2 nights in Luxembourg is too much, you might get bored. I would arrive there early evening and have dinner. Spend the night. The next day, visit all the sights and head off in the early evening to Paris.
Or you can skip Luxembourg and also go to Cologne!