I would have thought your husband would be able to tell you (or find out from the groom) exactly what they expect, especially if they are, as you say, very close friends, but hopefully the bride's sister (or possibly sister-in-law) can help you out.
I personally have never attended such a grand wedding here in Germany - I can't even remember if there was a dress code at any of my friends' weddings beyond something along the lines of "wear something nice" and "whatever you feel comfortable in, but please no jeans". And quite a few of my friends didn't marry in church anyway, but even at the church weddings I went to I can't remember seeing too many hats worn either by men or women, let alone top hats or fascinators (and scarves sound more Russian-orthodox to me, can't remember ever having seen anyone under the age of about eighty ever wear one in any kind of church service here, not that I attend them if I can avoid it). In one instance the groom wore tails, but that's about it.
Having said that, if the glossy magazines I peruse when I'm at the hairdresser's are to be believed, hats or fascinators are de rigueur among members of a certain set, and if the bride's last name is Mecklenburg means what Norbert Juffa says (I would have guessed something along those lines, but the name meant nothing to me) and your husband and the groom are Mensur Corpsbruders (I have similar feelings about that to those Qual der Wal voiced in #12) it is likely to be very formal and conservative indeed. And I agree with Qual der Wal, if your husband does intend to wear a morning suit he also ought to wear a hat, but since "dunkler Anzug" was also given as an option a dark elegant "normal" suit should also suffice. He could probably get away with wearing the same suit in the evening, but it sounds as if it is preferable if he changes into a tuxedo. If the wedding is the big affair it sounds like, you will definitely be expected to change into an evening dress, but like Qual der Wal said, you will likely have a few hours between the reception and the evening "do" in which you can go to your hotel room and shower, change, touch up your make-up, whatever.
Oh, and although most of my friends are people I know from university or else people I have met since but have been to university, I have luckily only attended one wedding where the groom was a Corps-Bruder, and certainly not Mensur (I came from the bride side of things). It wasn't a particularly pretentious or glamourous affair otherwise, but the other members of his Corps who attended the wedding all appeared in full regalia with band, cap, white gloves, one bore a banner with the Corps motto or something and stood behind the altar with the thing. Amused me no end, but they were dead serious...