That's probably what did it
Two big changes. The cage clean out followed by you being away. Did you take him to your gf's nan's house or did she come round to your house to look after him. The one time we took our last Syrian in his cage to someone's house when we went on holiday, he was extremely stressed on his return and started bar chewing. It took about two weeks for it to stop, and I ended up setting a playpen up round his cage, and leaving the cage door open so he could get in and out as he was so frantic. Eventually he stopped trying to get out (bar chew) when he realised he couldn't get any further than the playpen. But when we closed the cage door again he would start again. In the end we started going to bed early and turning the lights out when he started and he soon stopped when we left the room.
They don't take a big environmental change that well sometimes. If you think about it, suddenly his cage didn't smell the same and if he was at someone else's house, the house didn't smell the same and he freaked out possibly.
Even if his cage was left at your house, a different person's smell was around
Our last hammy was very sensitive to this and after only 3 days away once (we left him on his own for two nights, which is ok) he looked terrible when we got back! He soon got back to normal though, but I think he was pining! Some hamsters are fine being left for a few days though.
If you go away again, I wouldn't do a cage clean just before you go. Maybe just spot clean the week before. Doesn't matter if he gets slightly whiffy for a week or so. And then just spot clean a day or two after you get back, and leave a cage clean until he is more in a normal routine again
It has been very hot the last few days and that might be why he isn't using the wheel - our hamster hasn't been using his much either. They sit still to try and cool off I think (or just flop!) Do you have anything ceramic in his cage where he can sit to cool off? A ceramic mug on it's side on the substrate would do. You can even put it in the fridge for an hour or so each evening and then put it back in the cage when he wakes up at night.
There's a thread on here about keeping hamsters cool in hot weather.
I am sure he will settle down again soon, but try not to change or clean anything for at least 2 or 3 weeks now until he feels more settled. And keep talking to him. I guess a week or two seems like a very long time to a hamster
If the bar chewing doesn't settle then try giving him a bit longer out of cage time every night at the same time each night. If he starts bar chewing when you put him back in the cage, then you could try the going to bed early and putting the lights out! So later out of cage time might be better!
It might be an idea to persevere with the potty. Our hamster also used to chew his plastic one. You need to put it where he normally pees'. They won't use it if you put it somewhere else
So if you find his regular or main pee spot and put it there with the chinchilla sand in, and put a tiny bit of his pee'd on substrate on top he should start using it. If he is contrary, he might move it out of the way and pee where it was lol. If the plastic one isn't working you could just use a pyrex baking dish or something instead in the same place.
It really does help as you don't need to clean out very often then and just empty and refill the potty.
Hope he settles down again soon. As for moving the hammock (or was it the sputnik?). Our last hamster would get very cross and give black looks and be nervous and suspicious, if I moved something even an inch! Our current hamster doesn't bother quite so much. Depends on their personalities and levels of tameness possibly. They take something new being added much better than something familiar moving. He might have been sitting under it, as a shelter and suddenly it had moved lol.