"I checked Quest Outfitter (link below) and they claim the 1/2" tape weighs 3.5 grams/yard."
I wonder if that is the weight of the tape including the backing on both sides? If it is, it would be way lighter once it's applied. 3.5g/yd does seem heavy if it's for just the tape. Perhaps you could use it selectively (ie. the ridgeline) and sew in less stressed areas (ie. the perimeter hem). I've ordered some so I should be able to speak to the weight of this tape in a week or two.
"Dan...That is correct, the Zpacks Hammock Hex Tarp.
MLD as well as HMG both offer tarps of nearly the same dimensions (they are actually slightly smaller than the Zpacks), yet they weight more while being more expensive. I'm just worried about the durability of the Zpacks one since it is stitched."
Both MLD and HMG are using CT2K.08 (0.74oz/yd2) while Zpacks uses CT1K.08 (0.51oz/yd) for their tarps. That's a big part of the weight difference. CT2K.08 contains double the spectra/dyneema as CT1K.08, so it's significantly stronger. CT2K.08 is also about 5% more expensive which explains some of the price difference. The rest of the price difference is because thread costs almost nothing, while high end double side tape is somewhat expensive.
The rest of the weight differences would be to the weight of bonding vs. sewing, and due to the weight of the reinforcements and guyouts. I have an HMG tarp and it is very well reinforced with generous patches of heavier cuben. Overkill maybe...but it instills a lot of confidence in the tarp and it's hard to complain about an extra 1/2oz because the reinforcements are bomber. Looking quickly at the photos online, the Zpacks guyouts/reinforcements looks quite a bit lighter than the HMG ones, and MLD falls in between the two. I can't speak to the actual strength of the tie outs since I haven't tested them....perhaps all of them are plenty strong.
"I can actually make the tarp from Spinnaker for the same total weight (with rolled edges), and for half the price. Of course the only problem is that I can't find Spinnaker EXP anywhere right now."
I wouldn't....cuben is just so much better than Spinnaker and when you are making it yourself the cost difference isn't that radical. Cuben is more waterproof, way stronger, non saggy, longer lasting and isn't noisy like Spinn. For a MYOG tarp you are probably using 5-8 yards of material which means the cost difference is maybe $10-15/yd or $50-$120 total. That's money well spent IMO for a better tarp that is going to last way longer and be worth a lot more should you choose to sell it. Also if your skills are limited, bonding a tarp with double sided tape is way easier to learn than sewing.
"Should I really be this worried about the sewn stuff ...?"
Generally speaking, I don't like the idea of sewing two cuben panels together (ie. a ridgeline), but I don't have a problem with sewing stuff like guyouts as long as the area is well reinforced. On a guyout, if you've usually got the edge of the cuben folded over to be 2x as thick, and then you've got a reinforcement patch on there too, you are sewing through 3 layers of cuben at least so the stitch holes stretching isn't an issue. With stuff like ridgelines, it's hard to reinforce the cuben so the stitch holes don't stretch, and if you do reinforce it you are likely using bonding to do so, so why not just bond the actual seam?