first of all, congrats with your visa!
do you really want to work for a company that neglects their employees like this though? 2 weeks to get a response from HR department sounds really weird, they usually reply within the same day you message them. i know it's hard to secure a job 10000 km away from your home, but that's a massive commitment and it's better to be sure.
i've been in a situation like this, when company outsources visa related processes to a 3rd party and for me it almost turned a disaster
"Federal Agency of Employment" - assuming you've got a german visa and you still want to move over here but not quite sure about what's up with your employer : i'd continue my interviews with other companies, if you don't like this current employer when you land you'd still have time to transfer your visa to the new one. probation period works for both parties. if you go this way just have some offers in the pipeline. then you can simply reply "you know, i'm in germany already, just take over my visa". not a legal advice, please consult some more trusty source beforehand
i hope it will work out for you