Aniplex of Japan does not have the international rights to RE:Zero, meaning that Aniplex of America will not be releasing it over here.
Exactly. Physical media itself is now a niche market, and anime is another niche within that. Business wise, Aniplex is playing things very smart. They have several big name, popular titles under their belt, and the fans of these series are generally willing to pay more to own them physically. The ones who aren't just stream them.
It helps that they have pretty much the best image quality out of any company releasing anime in North America too! It's very rare to get a BD that's of the same quality as the original JP release from companies like Funimation or Sentai. They often have more problems caused by things like compression (putting up to 12 episodes on one disc can have a toll!) or different black levels (which can cause banding, or mess up the colors).
Comparatively, yes. You pay less, but get a disc that almost certainly has worse authoring. As for Funimation's limited editions, are you referring to their standard chipboard boxes? Their actual limited editions tend to be more expensive than $70. For example, their upcoming releases of Haruhi, Code Geass, and Escaflowne are $125, $120, and $150 USD respectively. Funimation is well aware that some people are willing to pay more than what the "standard" anime prices have been in North America, and they're jumping in on that bandwagon. Of course, their regular editions won't be going away; they'll stay as a more affordable option for people who want discs, but not in a collectors edition. That's the market Aniplex has no interest in, and seems to not be as big as it once was.
Mind you, those prices are still less than what Aniplex charges. It's just a difference in the audience that the companies are trying to reach with their physical products. Aniplex has no interest in getting their stuff stocked in Wal-Mart or HMV like Funimation does, so they can sell through specialty retailers and print far less copies. It's a small market, but one that's keeping Aniplex of America quite healthy.
This is often because a different member of the production committee sometimes gets to own the OVA. That adds another company for the licenser to negotiate with. Sometimes the company isn't willing to license out the OVA, or sometimes the licensing company doesn't feel it's worth the effort. I'm pretty sure this is why we're missing the director's cut of Gatchaman Crowds' last episode, but I could be wrong.