do either of you have any idea when this j was dropped from words such as "armija"?
I suppose you mean 'armja'. The spelling was changed in 1936 when it was decided that:wyrazy typu Maria pisze się przez i, z wyjątkiem po c, s, z (np. Francja, pasja, diecezja)
Many people pronounce Dania (the country) and dania (plural of danie) slightly differently and the spelling Danja shows that in writing...
In fact, most people, if not all, make this difference in pronounciation. Formally, the spelling 'Danja' should still be kept, even if it is against the above rule, in my view.
I've seen 'w tern' in some books
You could never see 'w tern' [w t-e-r-n] in books, you could only read this as 'w tern' whereas in fact it was 'w tem' [w t-e-m].
there are now two się's in the language that behave differently syntactically
What exactly do you have in mind ?