Good call. Considering the premise of episode 8, I bet episode 7 is about reuniting with Dawn and Brock in Sinnoh. They want that to be a surprise hence not revealing the title. If those really are a different Piplup and Craogunk that would just be downright mean. But remember these two actually are some of the few Pokemon to appear on both promotional posters, so they must be recurring characters rather than just filler mons. That alone makes me think they probably are THE Piplup an Croagunk.
That's also what makes me think episode 5's Snorlax is Ash's. It's featured so prominently on the first poster (next to the Kanto starters no less). That makes it pretty likely to be a recurring character as well, and if so would really only make sense for it to be Ash's. There's also the line from the blurb:
"Compared with Satoshi, the power is transmitted.", (referring to Snorlax). That translation is probably slightly off but it sounds to me like Snorlax achieves Dynamax through cooperation with Satoshi, much like how Mega Evolution worked.
I really doubt Episode 8 is related to Episode 7.
Episode 8 has the name of the region in its title. In previous summaries, when the name of the region appear in the title, it seems Ash and Go are arriving in that region (like Galar and Johto).
The Pokemon appearing in the poster could be just a reference that we will see these Pokemon in the early episodes. It doesn't necessarily mean they will be recurring characters.
Episode 5 is in Galar, then it's pretty likely that's a wild Snorlax.