I can tackle #6
Your assumption is correct. Triple tree held in place to the barrel with cross pin with eyelet (eyelet for sling attachement). The upper hole (shown in your photo) is an untapped hole. This upper hole is for screw that holds in bayo lug (threads in the bayo lug itself) or to locate detent for retention of a plastic close out cap (in lieu of a bayo lug)
Something on #4)
It appears that this model does not use the side magazine latch you have seen on the HK-91. Seems the mag catch mechanism is all internal to the receiver and it is activated by the paddle at the bottom of the magwell. The magazine has a lug on it's rear upper surface to hold it in the magwell.
More stuff here;
HKPRO: The MP5/10 & 40
Might be able to find out more here. Looks like a pretty active HK forum;
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