The shutoff of the whole die must be strong enough to take the whole clamp force of the molding machine. The inserts can be flush with the partline & vents made where needed to let out any trapped air that might cause a burn when compressed by th incoming plastic. The runners dont normally need venting unless very long and branched to multi cavities. I dont know what Q7 is but die cavity blocks must be sometimes have 10tons/inch squared of injection pressure, so for instance aluminum would be too weak unless extreme care taken by the moulder not to "flash" the die with too high injection pressure etc. I have never used Aluminum as a base holder block, but have used ii as the riser support blocks each side of the ejector plate with water cooling holes to conduct heat away from steel die plates. The A&B die plates must not bend apart when the injection pressure is applied to the cavity area, this is the critical thing to calculate, the thickness vs the length vs the force applied & distance between supports.
Hope this helps!