1) I believe Seafront is flats of 6 or 8 so maximum 8, almost certainly more than one toilet but it's likely you will be sharing. Don't worry about it though, I was in a flat of 14 in Penbryn with 4 toilets (2 bathrooms) and never had to queue. We divided it so 1 bathroom was for the girls and one for the guys.
2) Yes, there are many bus services all around the town and up the hill, the most important for students being the 03, the 301 and the 302. There are two bus stops for the 03 on the seafront, so no excuses haha
The 03 goes around town then up to Penglais campus and back, the 301 starts at the bus station, goes to Penglais then to Llanbadarn campus, down through Llanbadarn itself, to Morrisons, through Penparcau and back to the town centre. The 302 does this route in reverse, important if you're on Llanbadarn campus!
The bus pass is definitely worth it,
free travel on all MWT buses around Aber and discounts on bus travel out of town. A ticket from town to campus costs £1.10 (used to be £1, sad times) so if you're gonna use it more than a couple of times a week, the card pays for itself.
3) Not super ethnically diverse. Mostly very Welsh. The diversity is increased 100x during term time due to the large numbers of international students.
4) Can't help with Criminology sorry.
But I will say that you will love it in Aber, and especially on seafront, it's a beautiful place to live!