Can I marinate for longer than 24 hours? Something came up last night and I didn't get to cook this – want to make tonight, but it will have been...

I see no problem with a 48h marinade. IF you think you are going to get toooo much flavor that might hide the taste of the lamb, then take it out of the marinade after 24h and just wrap it in saran and refrigerate for the second 24h.

Absolutely you can use it - a good boneless leg of lamb is a terrible thing to waste! Also one of my favorites on the grill. Its flavor is pretty robust and forgiving, but I'd definitely cook it tonight, latest. (Also thanks for alerting me to this marinade - had missed it before.)