Not ridiculous at all, they're all good questions. This is just my take, as an elemental driver and someone that has spent many hours talking about elemental lore with Julie. I'm *not* laying down the law as an admin - others (admins or members in general) should feel free to contradict me and contribute to the discussion.
1) I think this one is easiest - as you point out glass is made from basically a lot of little stones, so would fall under Demin control. Indeed Spencer's wip Fiorella is based on this premise and she's a stone elemental.
2) I personally would say yes, especially in a mandrake's case since they are very much a plant and madin elementals can control plants. This is just my gut reaction though, I don't really have any argument to back this up. (Though now you've got me wondering if a stone elemental was petrified, whether they'd be able just to morph themselves back!)
3) Madin Bestia is an odd spell. I'm not entirely sure why an earth elemental is able to control animals. The general introduction to earth elementals in the guide says that Earth elementals are generally good with animals, but I read that to mean it is generally in the personalities to be good with animals rather than they have some kind of power over them. As earth elementals already have the power to control plant life, I think it would be rather overpowered of them if they could control all animals too - that would basically give them powers over any and every living thing.
4) Another gut reaction, but again I would say yes with caveats. I would suggest that an elemental would be able to control any spells that had an elemental nature of their affinity cast at them by a wand waver, but perhaps not actually immune to them? They wouldn't however be able to control those cast by fellow elementals otherwise elementals of the same affinity would never be able to do any harm to one another. I really don't know on this question - it's a very good point.
5) I say definitely yes to the first part - a fully trained elemental who is skilled at morphing can control what the form they morph into for any individual morph. I think this has been borne out in various threads by various drivers over time. Demeter's sheet lists a variety of forms she takes by habit in the appearance section and throughout her history has other forms she has taken mentioned. This was all approved by whomever sorted her.
Likewise, and tying in with my answer to 3) I would say that no - earth elementals cannot morph into animals. As you say, that would be the purview of an animagus. Also, the whole elemental slow aging thing is due to the fact that whilst they are morphed into their elemental state their bodies are in an elemental form and therefore not subject to the aging process. Animals clearly are subject to the aging process and so there's another thing that suggests.
Hope that all helps. As I say - this is all my interpretation.
