I think it's not as questionable as a lot of the other experiments as we get something out of it... well, it's still pretty traumatic, but it's not the usual load of filling scattered everywhere. It's also actually a tutorial/guide.
Shouldn't the result be a massive freak-pregnancy, with thousands of micro stalks growing in all directions, and/or deformed micro babies pouring out of the mamu-mamu?
Depends; I don't know if the jury has come back on whether reproduction requires specially "activated" bean paste; also, the presence of two paste cores just throws everything off.
Well, with the penipeni, it's supposed to penetrate into a certain structure, so I suppose part of that is what allows pregnancy to occur - the bean paste goes into a place where the yukkuri body can actually process it to start the pregnancy.
Whereas with this, bean paste and the paste core is just being mixed around, which results in more of an internal mess. As well, wouldn't doing this rupture the paste core? And wouldn't doing that alone reduce a yukkuri into a vegetable or something?
"Supposed to" being the operative word... Of course, Alices are special.
It probably would for a normal one, but Alices have custard, which is fairly soft, so it might not damage the cores as much. Using a Patchouli would lower the damage even more, except that a Patchouli likely wouldn't survive the initial joining.
I don't think extra filling would rupture the paste core- it's a fairly concentrated lump of filling around the middle, and the skin layer would stretch in order to accommodate it. If it did rupture, however, it'd probably go mad and also taint the genetic material.
As for the pregnancy, it depends; in most stories I've read, just moving paste from one Yukkuri to another doesn't result in a pregnancy but does transfer some of the characteristics over. I remember some of the pictures here indicate that a special paste "seed" is inserted during intercourse to start pregnancy.
There's nothing to suggest either of them has a proper paste core. Depending on the artist/author, the paste core is strictly a product of human intervention.
Also, even mixing the fillings would likely be enough to cause hybridization of some sort, and if an equal amount had been left in each one, they likely wouldn't have gone insane, which would have made for more interesting findings.