I think your thoughts are good :D
des Champs for this person and Schamp for later generations that used that name.
The underscore - des_Champs - comes in automatically when a two-name WikiTree ID is used (let WikiTree add it automatically though - I found out that it makes a mess if you add an underscore yourself when doing LNAB).
If there are other profile managers who might be interested in discussing this, you might want to post a link to this G2G question on the relevant profile.
Cheers, Liz