So the lady concocting the potions is asking for ingredients. If an investigator interacts with her, it asks, 'if an investigator has xxxx, you can discard xxxx'. I forget the exact verbiage, but it seems to imply that the investigator interacting with the chemist does not actually need to be the one carrying the ingredient.

We played it the logical way where the one that initiates discussion gas to be the holder.

Main Question:

Would it have been legal for an investigator without the ingredient to interact with the scientist, but a different investigator holds then discards the item?

It's weird that it's phrased so vaguely, but we definitely read it as the investigator holding the item must be the one to bring it to her and discard it. Basically, when anything is unclear from a game-rules perspective, it's best to default to common sense, IMO.

I don't remember the wording used in this specific occasion, but in general the key for "anyone in your party" is " any investigator ", not " an investigator "; this, and considering that in general specific items to deliver are always tied to the person speaking (talking in general in all the scenarios), I think it's safe the intent, despite the wonky language. Considering it's an app, I'd suggest you to submit the question to FFG so that if the wording's actually odd, it might get fixed for the next app patch