I'l echo what Heath has said, I received mine last weekend but won't have a chance to get out with it until late April. I'm VERY impressed by the quality and workmanship of this pack, it's as good or better than any commercial-grade pack, and from my experience that's not true of some cottage industry sewing.
My BV450 fits inside just fine, though I don't plan to carry that anytime soon here in MN, my Ursack is fine and a lot lighter, but to answer your question, I'm sure a BV500 would also fit inside - though it wouldn't leave a lot of room for your other gear. I can strap the BV450 on top with the single strap Ron uses, so again I assume a BV500 would work that way as well.
I'm not planning on using the inflatable pad for anything other than the pack's suspension. I've taken it out and deflated and inflated, sat and stood on it, seems every bit as strong as my Exped UL7 or Neoair, so no worries from me on durability.
I've walked around the house with 20 lbs in the pack and it was like nothing was there, I'm sure it can carry 35+ but I'll likely never exceed 20, but I did want the option of carrying more as I backpack with my kids as well. Let me know if you want any up close pics of anything on the pack. I HIGHLY recommend this pack based on its quality and design, but I can't yet comment on carrying it on a real hike.