Using 16 hole front rim to build a rear wheel I have the opportunity to buy 2 front Shimano Dura Ace C40 latest generation wheels at a very cheap price. The hole count for the wheelset is 16/21. Could I build / have built a rear wheel from one of the front rims, for example using a 32 hole rear hub or is 16 hole too fragile for a carbon rear rim? 

I don't have the hub yet, but from research I assume this is the most equilibrated option, correct? With these, do you think I would need new spokes or can I reuse the ones which come with the 16 hole hub? One of the patterns I saw could work, it doesn't require any crossing. Maybe the rear hub could interfere a bit though? The video corresponding to the screenshots supposedly is a professional wheel-builder as he was a making a project for a 16 hole rear wheel, tough steel / aluminium one.

There is also a review from CyclingTips comparing the new generation wheelsets with the old ones. There is a part where they say only the rear tubular version rim is asymmetric.
Sixteen spoke rear wheels do exist, but there aren't very many of them, and to function they need to have extremely stiff and strong rims. In this case, being a Dura-Ace product, it's reasonable to assume that the everything has already been pushed to around the limit in the design. In particular, making one of these a rear would require the compromise of either pushing the drive side spoke holes way beyond what they were expected to endure in terms of resisting cracking, or making the whole thing have way less tension than intended, resulting in it being weak. Just winging it and assuming you'll be fine is not a good idea here.

These limitations don't have to do with the rim being carbon per se.

Worth mentioning but pretty far down the list of issues with this plan is that the sort of rims these are (deep carbon rims in an ultra-premium prefab set) can have directional drilling that will contort the nipples around if a pattern other than intended is used.