The phenacetin likely not much of an issue for contact with water column if it's separated from it, only issue is if it somehow interacts with the catalytic process going on. From reading up on catalytic reactions and h2o2 taking the phosphoric acid content of this solution into account I have no idea if it's being used to adjust the pH of the h2o2 making it slightly more energetic at the 6% concentration under the conditions it is to be used in or if it's there to allow the phenacetin to remain in solution by making the solution more acidic, however it will likely be very small amounts and probably of a very low concentration itself as this product directs that it can be used as a mouthwash and gargle, phosphoric acid and teeth not the best combination as in higher concentrations its used as a dental etchant which is why it probably states not to be used for under 12's unless on the say 'so of a doctor', so probably not very safe to use in the presence of milk teeth, so likely a stronger concentration than what's in fizzy pop.
From having a look at the description for your device it appears that the catalyst used is a perishable item to be replaced intermittently so the presence of phosphoric acid may cause the catalysts longevity to decrease if it can eat the support medium that's holding the metal catalyst and because h2o2 catalytic reactions are influenced pH and phosphoric acid influences pH the reaction may be more energetic, but since the instructions direct you not to use 35% food grade solution which is very energetic and even give precise instructions on how to dilute to down to a 6% solution from 35%, then it's probably safe to use in the presence of phosphoric acid only that the reaction may just be a little quicker. There's a catalysed 35% h2o2 reaction over on YouTube that makes some interesting watching, although it's likely using a more exotic flavour of metal than the catalyst contained within the ceramics for this device.
If it's all you can get then if it were me I would test first and add a drop or two onto the catalyst to see how it reacts, if it reacts exactly like you expect it to compared to that used previously then it's probably safe to use for the purpose you intend. A more energetic reaction just means more gas produced (and heat of course), if it's too quick then it may produce enough gas that the container can't cope with the quickly expanding volume and fail, I can't see that happening though at the 6% concentration.
You could try Boots, you'll need to ask at the pharmacy if it's not on the shelf, they may have some 6% h2o2 that comes without the extras you'd rather do without and take the doubt out the equation.
