[RPG] Can ghouls exist on the Material plane

You're right, you're kind of stuck.

Thankfully, the action economy of 5e works in your favor here.

Here's what you need to do:

• Select a crystal as your Arcane focus
• Wear the crystal on your body (necklace, pendant, whatever). Components pouch also works here as there is no action economy on drawing stowing one of those.
• When you want to cast a spell, drop your weapon and touch your crystal
• When you're done casting, pick up your weapon.

This takes no additional actions, is perfectly legal (nothing specifies how big a crystal must be, just that you have a hand free to use it. (dropping a weapon costs you nothing, picking it up is your interact with object that's free with an attack or move).

The only real risk here is that if you do this a great deal, an enemy might begin to ready actions to attack you, or even move away from you while your weapon is stowed. This is something you'll have to live with for this to actually work.

Last thing that I'll say is that you should talk to your Adv League GM, or even seek out an answer from their twitter feed on this. GMs for Adv League are supposed to be lenient and use common sense in their rulings as the ruleset is flexible by design.

This sounds like a problem with the adventure.

A bit of a frame challenge here, but I feel like this is more a problem with the adventure than the party. 5e doesn't have enough material that you can assume that every party can solve every problem (like you could in 3.5), and assuming that a 9th level party can affect the ethereal plane isn't a reasonable assumption in a normal 5e game.

The problem is that seeing into the Ethereal and affecting the Ethereal are very different things. 5e doesn't even have the old standby of force effects working against Ethereal targets like 3.5 did. It's going to be pretty difficult for a 9th level party to be able to do anything meaningful to an Ethereal target.

Given the restriction of not being able to add Ethereal-affecting items to the existing adventure, here's what I would suggest:

Give the enemy a reason to attack the players.

As you note, the tactically correct option would be to go Ethereal and wait out the party, which would make the party lose no matter what. A way to sidestep this issue is to give the enemy a reason to attack the party despite the fact that it could safely wait out the combat. Is the enemy non-intelligent, like an animal? Does the enemy have a vendetta against the party, and wants to kill them beyond reason? Does the enemy have a goal that involves the death of the party? Is the enemy just dumb, and doesn't think of using his abilities to their greatest effect?

If the enemy has a reason to come out of the Ethereal to attack the players, and uses their Etherealness to improve their combat ability rather than avoid the combat entirely, you're going to end up with a fight that's likely to be more fun.