But then you are assuming this is all happening off-screen that she is using more drugs than she should be but we've seen no sign of her doing that apart from those times we've seen her doing it. For a plot that big you'd think we'd have at least seen more of this drug use from her.
Quinn freeloading on her own drugs is admittedly very speculative. I agree we haven't seen her using them very often. Then again, how often have we seen Riona or Tommy using them? The only time we saw Riona using them alone I can think of off the top of my head is when she meets the MC on the deck in Episode 5. There's a lot of implied drug use, but it's short on details.
I think the case for Quinn lying to Tommy is a lot stronger, though. We saw Quinn talking to Riona about the shortfall ourselves. Not only does she never mention Riona's freeloading (much less promise to keep it a secret), Riona herself asks how they could be short. Unless Riona is actually delusional, why would she ask that question when they both know the answer and won't talk about it?
So for me, the question isn't *if* Quinn was lying to Tommy but *why* she was lying. Quinn using too many drugs herself is the simplest explanation IMHO.
The alternatives I can think of require a lot more assumptions. From most to least plausible: Melanie and Sarah are taking the money for their own purposes; Quinn has an extra-secret agenda she is using the money for; Quinn is even worse at math than Dawe; a third party is stealing the money somehow. Those are possible, but I would be more surprised by those than Quinn freeloading.