I... Didn;t know Samus Returns had a fan devide similar to the great Awakening Casual Mode War. And, yet again, it's about an optional thing in the game, the Scan Pulse. I personally love the option because it helps navigations and fills out the map while not pointing Samus her Arm Cannon at your head to use it. It's a quality of Life convienience at best. But, like Casual Mode in Awakening (even though Awakening is being the Takumi for New Mystery of the Emblem) there are people who claim it removes all challenge the game would have, just by allowing you to fill up a map and ocasionally knowing where certain fake walls are...
Personally, I'm not really a Metroid Fan yet, as I've only finished one game, though that one game I loved and got me to want to expirience all the others I can (so, no Other M and Prime Trilogy yet for me). But, yet again, the first game in the series for a long time gets downplayed just because it has an OPTIONAL feature some people dislike. What's next? Gen 8 is gonna have an optional function where you can flick your wrist to toss a Pokéball like Pokémon Let's go which you can completely ignore and that's gonna get torn to threads?
Greninja is a great choice, and definitely the best water starter in Rejuv! Here are some of my favorites in the game I'd recommend looking into:
1) Lycanroc Midday: access to powerful STAB moves like Accelerock and Stone Edge make this a formidable foe
2) Sylveon: Don't remember which one, but I believe Eevee is available after one of the missions for the side quests
3) Alolan Muk: It takes a while to get going, but Alolan Muk is a fantastic wall in game with only one weakness
4) Trevenant: This may be slow, but access to the moves Will-O-Wisp and Leech Seed make for an interesting mon to use
5) Manectric: There's also an argument for Alolan Raichu here, but personally, I find Manectric to be much more reliable
6) Swoobat: this is mainly a pick you use if you're patient, but this slot is specifically for Simple Swoobat. Getting insane boosts from moves like Double Team and Calm Mind make for a combination that can potentially end whole gym leaders
7) Pyroar: Don't remember where it was moved to, but I've used one on my main team. It's got a decent Special Attack stat, and Noble Roar comes in handy against some bosses. I'd recommend Unerve for the ability instead of Moxie
You're not stuck. Your problem is:
You're going the wrong way. The gate doesn't open, and I don't believe it can be bypassed until later. Go back to the lab you crafted the mirror in. Outside of that lab you will be in the black version of the cavern. In that room will be a trio of glowing stones embedded in the wall. Interact with those, and shine the Darchlight Mirror on them. That will transform the cavern into the white version. There will be a bridge across the chasm in that version which you can cross to find your way forward.