1. "Christian sunna" (also known as "hadith of kafirs") is when someone does not follow the islamic tradition - for example erects statues (like spanish umayads) or in opinion of some uses paper money (rather than bimetallic islamic currency system). Handguns were considered "christian sunna" in an extremely orthodox (and in the same time extremely unislamic) mamluk society, i.e. they were considered a chirstian novelty that is prohibited for proper muslims (source - Ayalon quotation of Tuman bey's speach).
2. I'll find this article, but it will probably take some time - I'll have to check out the books from the library, and right now I'm on a little bit tight schedule.
3. As far as I remember the decision of qadis that permitted the use of crossbow was made during the rulership or Calawun, (14th century) or somewhere in this range of time, centuries before the establishment of a portuguese base in Oman. The problem is that crossbow is nearly always referred in islamic sources as a "bow", the paper makes the point in emphasizing that it's (as far as I remember) sometimes is called a "persian bow" or "the bow where string is drawn with the foot". The size of these bows is reported to be completely enormous - elite, extremely strong mamluks were rumored (according to the paper, that I'll dig up
) to use bolts that weighted up to a few dozen pounds and more.