My gut feeling, for midwing, 6mm AL is pretty thin -- borderline, but probably on the OK side. If you can get steel strip-stock or rod stock in that size, do so.
As for an indexing pin, I'd lean toward a 3mm-ish rod-stock or hardwood dowel cut to about 3cm long. If you have any long screws with a solid shank near that size, you could chop them up for that smooth shaft. Glue one end into the center section and have a mating hole on the other side. sand the peg on the un-glued side a bit so the pin slips in relitively easily -- you don't want slop, but don't want to have to force it either.
I typically get my thin ply from one of the local hobby or craft stores. Not sure what you have in your area to recommend similar. Our common soft craft wood (heavier than balsa, but softer than most practical woods like oak or pine) is "basswood" . . . I wouldn't be surprised if it wasn't available in your area, but that a similar wood is probably available. HARD balsa would do, but it can be difficult to find at times . . . and sometimes you can't find anything but
For 1.5mm balsa (what we call 1/16") you can generally see soft/hard patches by holding the strip up to an overhead light. See light clearly shine through, it's soft. See a light amber glow, it's medium. See none, it's hard. For this application, you need something relatively thin and durable to make a clean end-cap. For most of your ribs, ideally, you'd lean toward medium balsa, but your facings you just want a bit more durability. All else fails, hard balsa will do, but if you can, you want something just a little harder than that.
If you want to use only packing tape, that should still work fine. I'd still wrap the ends of the wing with packing tape separately to give the joining tape layer something to hold on to that it won't rip up when removed -- the acrylic glue common on packing tape sticks far too well to covering materials . . . but ironically, not as well to itself. the first layer of tape gives a bed for most tapes to grip lightly and release easily when pealed off after the flight. In flight, the lateral forces on the wings are very low, so you just need a strip all the way around to hold the joint flush.
As far as rubber bands . . . all depends on where and how many. Bands can hold the tips on, but building an aerodynamically clean locking mechanism may be tricky. For wing-mounting, I've held on a 3m wing before with bands (6-8 of our #64 bands worked reasonably well), and I've held them on with hard-points (pegs and wing-bolts). The hard points are easier to work with come assembly/disassembly time, but the bands are more forgiving when you catch a wingtip on landing.