Chaining Mareanie is unfortunately a very long and drawn out process; you'll need a lot of patience and prior preparation if you want to chain a Mareanie with any particular attribute -- whether that be an ability, nature or amount of IVs. If it's any consolation I chained some Mareanie myself a while ago, so I can offer some insight into some ways to make the process a bit faster or more convenient.
For your first question, not only does it depend on where you encounter the Corsola, but also on the means through which you find it. These are the numerous ways through which you can find one, and the chances you have of getting one to appear in each:
- On Route 9 on Akala Island, there is a single fishing spot in the south of the route. If you fish there, you have a 5% chance of downing a Corsola. If the spot is bubbling, it will make no difference.
- In Melemele Sea, there are numerous fishing spots you can access whilst surfing. If you fish in them, you have a meagre 1% chance of finding a Corsola -- you'll just find a bunch of Magikarp doing this.
- The third and best method is also on Melemele Sea, doing the exact same thing except making sure to fish only in bubbling water spots. If you do that, you have a decent 20% chance of finding a Corsola.
So obviously you'll be making your way to Melemele Sea to fish in some bubbling spots. What if there aren't any bubbling spots? Though my past experience, I found that whenever I left the Melemele Sea area (thus returning to Route 1 or Hau'oli City) and then returned to the Sea, all the bubbling spots were reset and reappeared. You can take advantage of this by fishing in a spot right by the border between the Sea and Route 1, so you can keep resetting the fishing spot. The best spot for this is toward the bottom of the area, and allows you to access another bubbling spot almost instantly after you end an encounter or find an item while fishing. Also, be sure not be holding B on Lapras or Sharpedo while near the fishing area, as doing this will cause the bubbling spot to fade instantly.
Regarding your question about the Adrenaline Orb, unfortunately it seems that this info isn't available on the internet yet. However, from past experience, it seems that Corsola will call for help very frequently after an Adrenaline Orb, and almost always if you can knock down its health. This is backed by one of the few pieces of information we do know about SOS battles, which is that each Pokemon has a "call rate". Corsola's is ranked at 9 -- for context, the next and highest level is 15. So by comparison you'll be getting encounters quite often while chaining Corsola.
So fortunately, you won't encounter too much anguish actually getting Corsola to call for help, but you might when it comes to getting an actual Mareanie to appear. Assuming something does actually appear when Corsola calls, it'll only be a Mareanie 20% of the time, which means if you're unlucky, you might be knocking out Corsola for quite an extended period of time. (Also, if for some reason you decided not to use the bubbling spots on Melemele Sea, now might be the time to switch. Without a bubbling spot, Mareanie's call rate is knocked down to 1% and 5% on Melemele Sea and Route 9 respectively.)
Unfortunately, I can't give an exact answer to your question about the chance Corsola's call will fail. However, with experience using this mechanic, I believe that a. the chance it will fail never changes and is never different and b. the chance it will fail is around 1/3. This, this, this and this are probably the best resources for information on SOS chaining, so if you have any further questions, I'd recommend checking those out. Also, Serebii lists all the SOS battle Pokemon and links to location pages where their encounter rates are listed, so I'd check that out too if you're in need of more numbers.
Also, a few tips for SOS chaining to help the process go by a bit faster:
- If you need to kill a turn because the Pokemon didn't call for help or its call failed, use another Adrenaline Orb or a Revive on a Pokemon that isn't fainted. It will waste your turn, and you won't lose the item.
- If you want to get a certain nature on Mareanie, use the ability Synchronise and make sure you're chaining with them active in the battle so that their ability affects called Pokemon too. All spawned Pokemon will share the nature of the "synchroniser" if you do this. Trace is also a useful ability to see the Mareanie's ability without catching it and ending the chain. (You can also see if it keeps getting crits after it poisons your Pokemon with Poison Sting, or if Thunder Wave fails on it to check for Regenerator.)
- Mareanie will attack the Corsola that you called in before it attacks your own Pokemon. If Corsola is at low HP, the Mareanie may knock it out, which is bad since Mareanie's own call rate is 0 and so your chain ends right there if this happens. Sources on the internet suggest that below 50% health is enough to get SOS chains flowing, so False Swipe might not be best if you're looking to chain beyond the first Mareanie (ie. to the 30th chain to maximise the chance of getting a Hidden Ability).
- Contrary to what some sources may describe, it is perfectly OK to knock out the first Corsola you encounter, so long as it already has another Corsola as its ally. You might end up wanting to do this if the Corsola gets too many boosts using Ancient Power. The ally Corsola will continue calling for help, and it will not reset your chain.
I hope this answers your questions! Let me know if you have any more questions.
