It is great news that Nonsense is feeling more confident and coming out. Good going!
Getting Timber to go back in the cage when I have to leave was a struggle here too. That's just part of it. They understand language, so be sure and use the same phrase when it's time to go in. For instance, with Timber it is "time to kennel up Timber" (thanks to my boys). I also tell him what I'm doing, like "I have to go to work, time to kennel up Timber, I'll be back." Or "I'll be back in an hour, time to kennel up Timber." In time, they learn that going in is a temporary state, you will be back, and they will be out again. Same thing at night, "time for bed, kennel up Timber." I always try to communicate what's going on and they learn exactly what those words mean in time. You have probably already noticed that they are very intelligent.
That said, even now there are times (they are rare but happen) when I'm really in a hurry and don't have time to coax him and he doesn't want to go in for whatever reason. In the early days, like you I had to threaten the towel (he hates it). I never had to use it, I just keep it in a corner and would go pick it up and head his direction. He would head inside immediately. Now, all I have to do is say "kennel up or I'm getting the towel" and he goes in. He knows I'm serious, because I don't use it often. Over use would make it an empty threat.
Most greys love routine. You have made a good start with putting the treat in the cage. Try continuing to do that with the phrase you want Nonsense to understand when it is time to go in. It takes time and patience but you'll get there!
Other members will have more ideas also I'm sure.