"Another aspect of it is that I'm a marketing director - I know how marketing works - and I know that Father Christmas was invented by the Coca Cola company rather less than 100 years ago " - See, that's the problem with marketing people - they assume that everything is a marketing ploy. Have you heard of the poem 'T'was the Night before Christmas'? It was published in 1823, which is rather more than 100 years ago. Rather undermines your argument.
I don't give a monkey's as to who believes in Father Christmas or not, and the actual decision as to whether or not FC 'visits' your house is valid, whatever the outcome. The philosophical reasoning behind the OP, however, is deeply flawed. If you're going to be consistent with your argument that you won't go along with the suspension of disbelief, you should also ban your child from dressing up in a space helmet, not allow them to act in any plays, not read anything other than non-fiction, etc. In fact, you come across rather like Mr Gradgrind in Hard Times. Saying you just can't be arsed would be less pretentious and a hell of a lot fairer on the children. By telling them FC does not exist, you're as bad as the people you deplore - you're depriving your children of the choice to believe and the decision-making process.
If you really were that devastated at finding out FC didn't exist, and want to protect your DD from the disappointment (though I suspect she will be disillusioned in many more hurtful ways over the years than finding out that actually, her parents bought her some presents - her 'best friend' saying something nasty about her, breaking up with her first boyfriend...), then fine, that's your choice; I feel that disillusionment is a necessary part of growing up and managing the process is the parental responsibility, helping children to rehearse future situations ("you're right, Father Christmas doesn't exist - how did you work that out? That's very clever reasoning, well done. So who did give the presents? Why? Do you like surprises? Do you want to carry on pretending, or shall we make up something else now that you're bigger?"). Have you considered the opposite scenario - that she will get to an age when she realises everybody else gets stockings, that they all know, or think they do, that it's really their parents, and she comes to the conclusion her parents just don't care or love her enough, which is what happened to a friend of mine? Just a thought.
If my own DD asks questions, I'll make up stories or tell her truths or half-truths depending on her age - same as I do for everything else. Every time she asks why it's raining, I give her a different story. She's 2.4, she really doesn't need to know about condensation/evaporation, convection currents, etc yet, though that's one of the stories I tell her. TBH, for her the thought that the water is magically drawn up out of the sea and makes clouds which blow inland, hit a hill and then turn into rain is as fantastical and enjoyable that it's her 'special dance' which caused the rain. Later on she'll decide which seems the more reasonable explanation.
As for 'tat' - I'm with you on no tat, but really, it's not FC who chooses the presents, it's you. If you don't want tat in the stockings, don't buy it... DD's getting some Happyland farm animals and people wrapped up individually and a couple of Brio trains dismantled and wrapped separately. And some hairclips, which probably do count as tat, but we lose them at the rate of one a day...