ad1 that is correct, (if there is more than one german dictionary the kindle will choose just one randomly.)
ad2 not quite- there are two language metadata entries in a prc or mobi file.
The first one you can manipulate via mobi2mobi.
This is the one that determines which language the kindle will use to look up a word in that book.
If the book is a dictionary however, there is an independent entry, determining the language of the dictionary- this one you can manipulate via mobipatch. (in former fw versions you had to set this one to english, in order to use a dictionary as primary dictionary- but this no longer applies)
so... you can use mobi2mobi to change the metadata in your bookcollections
you can use mobipatch if there is e.g more than one german dictionary in your collection and kindle chooses one you don't like for instant lookup
however keep in mind that many books aren't tagged correctly. I bought a german book from amazon, where the metadata entry says english... and there is no way to manipulate them as long as they are DRMed