This episode was defiantly dialogue heavy, so it's hard to give opinions on what happened and the flashback with Koharu and Yamper until subs come out, I just know Yamper being jealous of Koharu being nice to Pidove makes sense as Koharu was very cold to Yamper, who wanted Koharu to appreciate and love it. But I need to wait for subtitles to fully confirm what actually happened, I also want to know what was Koharu said to make Sobble stop being invisible during the sequence which lead to flashback showing that Koharu and Yamper first didn't hit off well.
Ash's Dragonite calming Sobble down doesn't 'prove Ash is the better trainer for Sobble' all it did was show that Dragonite has the capabilities of relaxing anxious Pokémon with its hugs. Sobble is perfectly happy with Gou (Gou gave Sobble some confidence in the last episode) however it will take time for Sobble to get over being timid and overwhelmed in crowds. In fact a Pokémon of Ash helping a Pokémon of Gou's parallels how Ash often helps Gou, the former acting like a mentor to the latter, so if Ash had Sobble, then that comparison would have been completely lost.