I'm considering giving the AR platform another try, not super crazy about it, but could be with the 7.62x39 upper. I've done some research and the biggest issues is the mags. So if you're going to build one build it with the lower that accepts the AK mags, GRANTED !!! it don't look as sexy but they will feed more reliablely. DTI (delton) has a complete kit - lower for like 460 I beleive. The rifle you have pictured at the first of your post has a Vulcan V58-V68 receiver ,I'm not sure witch on the modle #, that does accept the AK mags.
AK mags are really the only way to go due to relailabity issues, cost, and availibility. Ive priced some of the stenag 7.62x39 mags, aside from being scarce they are expensive, even the franken mags (about 2" from top AR mag, the remaining bottom AK mag welded togather) are kinda pricey. On the transverse what does an AK mag cost ??
The only problem is that the AR lowers that accept the AK mags are alittle difficult to find. Good luck, please post pics if you build/buy one