I don't quite get what was wrong with sitting at one end of the table, but if it was all rearranged and everyone was happy, then great! I wouldn't take it on as a deliberate snub - when we have a family get together it's every man for himself when it comes to seating, no set order, no meaning put on who sits with who. There's no such thing as a 'better' seat or a 'worse' seat - we are all family sitting together. Perhaps the others at the meal didn't see the seating arrangement as a status thing, as you seem to.
Problem 2 - you can advise but she needs to be a big girl and deal with this herself. It sounds like a huge reaction to 'teasing'. Is it teasing, or bullying/being nasty? I think it shows character to deal with teasing/banter in good humour (but then I'm British, it's our bread and butter) but bullying and nastiness needs to be challenged. I'm sorry but I have no idea what you mean by she doesn't feel 'honoured' by them. Her (husband? I'm a bit confused) can talk to his mum. She may not realise she is crossing the line. If he isn't convinced, if he thinks his mum is being deliberately nasty, then he needs to step up for your daughter and put a stop to it, firmly.
Overall I am wondering if there is a bit of a culture clash (or two) here. Even cultures from different parts of the same country can be quite different. Your emphasis on status being reflected by where you sit, and the concept of someone 'having a place' (as in heirarchy?) in the family (although generally speaking our oldies are higher status than the kids, of course, and I think that's pretty universal? But at the meal you were offended that the younger members (DH) were in a lower status seat than their elders (their aunts/uncles), and 'honouring' someone, are so far from my experience I find it hard to understand your problem. If they are also not the values of the other people you talk about, then it may all be a big misunderstanding.