I've been thinking about the possibility that something has happened to Liones in the last 6 years. The way that Sin always talked about their destination gave me the impression that it was relatively close, but in NNT Liones was almost on the other side of the island compared to Camelot. There was also a potentially suspicious detail in chapter 24 when Sin showed the group a map of southern Britannia. A section of the map was ripped and Sin had his paw covering the rolled up paper so that the area just after the tangled forest wasn't visible. This could just be a case of Nakaba wanting to hide future locations from the audience, but I could see a scenario in which Liones was relocated closer to Camelot for some reason. That would also make Howzer's trip to Cant way more reasonable, as it would be a bit weird for the captain of the Liones knights to just randomly go visit a town so far from his home. If Donny and Edlin were being trained in Liones, the kingdom being relocated would also explain how they ended up so far from the kingdom despite being so weak.
Interestingly, in chapter 6 Sin mentioned that Liones was about 200 miles from their current location, which was near Paysan. If you look at the map of Britannia that shows the locations of Liones and Camelot, and the map of southern Britannia that shows the positions of Camelot and Paysan, and then compare the two with a map of the UK, it doesn't look like Liones is 200 miles from Paysan. In fact the distance is almost double that assuming they have to walk there. Roughly 200 miles would actually put the party right in the middle of the continent, which just so happens to be close to the section of Sin's map that wasn't visible. That's pretty suspicious, but in fairness it could be that Britannia isn't the same size as the actual British Isles, or that Nakaba botched the numbers, or that 200 miles was mistranslated, etc.
I have compiled a little graphic to illustrate my point. Graphic design is my passion and so on and so forth.