The jar I got specifically said on the label it had mother in it, so just look for that. It said something to the effect of "the sediment in the jar is the mother". There was not some huge floating blob or anything like that in the jar, nor is there in my jar. But it's definitely vinegar, the aroma is really powerful.
I don't really know how long it took, I just dumped stuff in, checked it after a week or so, then forgot about it for a month or two. When I checked again it smelled strongly of acetic acid, nothing really noticeable after a week.
I'm sure a larger mother will work faster, there would be more bacteria to convert the EtOH to acetic acid.
Malt vinegar is an interesting thing to think about making - I never really have any leftover beer to add to the jar
Plus I don't know what affect the hops would have on the acetobacter. But you could pull some off after your mash and before you hop it, ferment it separately, and then add it to the jar. That would probably be much better than trying to make it out of something like BMC, I think you want a full-flavored malt beverage to go into making the vinegar. I'm having visions of a RIS vinegar that sounds delicious . . . or imagine how using a Belgian yeast strain to ferment the beer before feeding it to the mother . . . I might need a lot more jars