“As you can see, I had put the potty dish where Serendipity has since suggested he'd likely build a nest (so it seems maybe a nest is being built on top of the potty dish!”
This is very likely! Yes I would move the potty dish to the centre back compartment. Empty it out first and put new sand in. If you can save a tiny bit of nest from it then out that in the front left compartment along with a smelly bit of food (cheese or cucumber) - just a tiny piece. I’d also put a small handful of hamster mix in the front right compartment. This will give him the idea to hoard in the house. Although they usually bury their hoards under the nest. Yes I’d take the blocked up tube out.
Nesting material - not sure what you have next to the house entrance but I’d replace it with plain white toilet paper. Tear a few sheets into strips and put them next to the house entrance. They love it as it’s soft (and also safe) - I had one hamster actually weave the strips into an amazing round nest with a hole in the middle! Usually they do arrange the longer strips into a cosy nest but also take bits of soft substrate in there too to pack out the compartment.
I think he’s just gone to the darkest corner he could find and hasn’t quite discovered the house yet. You could also take a bit of the substrate from the blocked up end of the tube and put that in the nesting compartment too - it will smell familiar snd he’ll associate it with “nest”.
It’s a great set up as Ria says. Another thing they do in the first few days is explore every inch of the cage to see if there is an escape route! Which may be the digging at the sand and hemp mat.
You could also move the bendy bridge (the one that’s over the tube) over the house entrance - tunnel tempts them in - and it gives easy access to the house roof as well.
One tip with that house though - is to avoid taking the roof off at first if he moves in. This can scare them into abandoning the house. They get used to the roof coming off wfeg more confident but avoid the temptation to keep peeking. You should know if he’s moved in - you’ll see him come out of it. And the toilet paper strips gone (add more). Also you might hear a lot of scratching and banging in the house. I have no idea what they do or why they do that but it’s like they are renovating! Probably scent marking and testing things out.
If he does move in he may go awol for a few days and stay in there (only coming out at night). But don’t worry - that’s just temporary.
To help with the frustration of the first couple of weeks I have a hamster cam
. It’s just a small indoor security camera that records and streams to your phone - infrared. It’s great fun - you can sit in bed and watch what he’s up to after dark - or replay the video next day. Costs about £30 and takes a mini SD card.