That's very interesting. In my only semi-professional capacity, I'd say it would be unlikely for anyone to be able to force someone to swallow a piece of mushroom without there being some mushroom caught on the teeth. If Meredith had swallowed it voluntarily, as you say, she would have had to have been killed within a ridiculously short time for it not to have reached her stomach.
My big question there is, was the mushroom raw or cooked, and was there any mushroom lying around in the flat? I could see someone just returning home, and seeing some raw mushroom, snacking on a small piece. Or even, for some bizarre reason, a murderer forcing his victim to swallow a piece that was close at hand (though bear in mind what I said about the teeth in that case). Either scenario seems very implausible though.
If the mushroom was cooked, and if there was mushroom in the pizza (pretty likely, but do we know?), then it seems to me to be a no-brainer that it was partially regurgitated during the struggle, or even slipped into the oesophagus through the relaxing cardiac sphincter after death.
Which merely underlines the fact that the time between eating the pizza and death was relatively short.
Rolfe.