Just take it steady with things. Hemp substrate is good - I think the Hugro one is better than the Siccofloor but I've only tried the Siccofloor (I was allergic to it!) but I wouldn't switch substrate just yet, wait till he's settled down a bit and got used to the recent changes. What substrate are you using now? It's ok if he hides in the playpen - he doesn't need to be running round enjoying himself. The main thing is that he can get in and out of his cage himself into the playpen so he has no reason to chew the bars right now. It'll help break the habit. Not sure what you've got from Zooplus, but those long bendy tubes could be a good way of giving access from the cage to the playpen (is the cage on the floor?) depending on how big your cage doors are. The tube would probably wedge right into one of my smaller doors, and then if it's extended it could lead into the playpen opening up there at the other end of the tube. If that sounds like too much hassle then just do what you're doing.
Our hamster tended to just go behind his cage and have a wash lol and sit there for ages. He thought it was a great game if I tried to catch him and he would run backwards and forwards behind the cage where I couldn't reach. This was all within the playpen though.
The chamomile flowers - just sprinkle a few - maybe 5 or 6 - on the substrate - not too many.
As long as the long pointy bits of the zip ties are on the outside of the cage they shouldn't cause a problem although I tend to snip them off (do you mean cable ties or the ties that you put round plastic freezer bags?) The locking cable ties can be snipped off - as close as you can so it doesn't leave a sharp edge). The others I call twist ties.
He's probably still very nervous being so young, and will get scared easily and want to hide, but he might find something more interesting to chew while in the playpen