I would split them up as much as possible, if anything because that would make it easier to monitor tivs for a few days after you lock to see if tivs went up and cancel/resubmit if you could get a little more for your trade. I don't think there's any intrinsic risk in packing them all together though if that's what you prefer. You may want to take a couple pictures beforehand though just in case.
Yeah, the thought of the box getting lost is not a scenario I would handle very well. I guess between 5-10 shipments would be ideal if I want to combat losing everything in one swoop and making sure the TIV doesn't fluctuate. I don't see the TIV being an issue though because I stopped purchasing games like 7 months ago.