I'm afraid I don't really think you can lay 100% of the blame on the dealer here... there were two of you involved in the transaction, and just because the horse you bought turned out to be unsuitable for you, doesn't mean it was completely his fault! Everything that you did with the horse at the dealer's yard, it behaved well and you were happy with it (clearly happy enough to buy it!), so can you really blame him for the issues you had when you got it home? Most dealers simply do not have the luxury of time to get to know their horses inside-out like private owners do... they can only tell you what they know. Plus, nobody (dealers OR private sellers) has a magic crystal ball where they can see how the horse will behave in its new home. Quite frankly, it was just as much an error of judgement on your part - you are self-confessed novices, yet you went to try out strange horses without an experienced person to give you guidance... not to mention pissing off on holiday right after you bought it, I'm still a little shocked you would do that tbh, and you have unfortunately learned the hard way that buying a new horse is not quite the same as buying a new car or washing machine...!
Regarding the solo hacking - I am not doubting the ability or knowledge of the experienced rider who has given you their verdict, but really, if you wanted a horse that can hack alone (with a novice rider no less!), why oh why did you not TRY hacking it on its own BEFORE you decided to buy it?
Next time you go to view a horse, make sure you take someone with you who knows what they're doing (...the rider who's been helping you sounds perfect, someone who can not only give you their opinions etc but also someone who get on and put the horse through its paces in a way that your daughter is not skilled enough to do). And think a bit more carefully about the sort of situations you want to assess its behaviour in... if you want a horse without separation anxiety then take it completely away from other horses (and/or take other horses away from it) and see what it does... if you want a horse who can solo hack then take it for a solo hack! The onus is on YOU to test it out thoroughly, it isn't the seller's responsibility to tell you what situations to try it in. Think about what you want to do with the horse, the environment /routine you will keep it in etc. and then TEST it in those situations so that there won't be any nasty surprises when you get it home!
These days with Facebook etc it is SO easy to find out about horse dealers there really is no reason to end up using a bad one. Ask for personal recommendations from people who have bought from them and are HAPPY with what they bought - and specifically look for other novices like yourselves as those are the opinions that are most valuable to you.
As you're looking for a horse for a teenager I would also recommend contacting local Pony Clubs and asking them if they know of suitable horses for sale... good childrens/teenagers' horses tend to be sold by word of mouth and PCs are the best places to look for them.