Losing a 0-level spell (or any other level for that matter) to command: Drop, and later cast sanctuary or invisibility...'tis fine. The DC for the command might be based on the level of spell 'lost' rather than the HD of the Sylph, but that's not horrific, just a minor nit-pick. The DC for the sanctuary should also be taken from the spell level 'lost' (or the HD of the Sylph, no big deal really) so that it's actually useful when you get it...
Edit: Actually, unless the Sylvan Defender gains spellcasting levels, the DC should remain HD based.