I just threw this up in another post, it will answer a lot of questions you have!
In short though if you use the O&G army book then you would use full O&G animosity rules as well as get the option to upgrade one unit to Big 'Uns with the normal restrictions (ie you need another unit of normal boyz before you can upgrade a second unit).
Yeah the one thing I really would have liked to see in a 6th ed book is more magic items. Their RH items are often very situational and you never have the depth of options that others get in full 6th. I’m thinking of coming up with some options on my own in the future for casual games, I think it could be a lot of fun.
As for a Lore of Hashut, take a look at the spells from 4th edition! You can at least get the flavor, if not their actual use haha