I think your questions re: the timing and influence of Irenaeus are good ones. They do militate, in my opinion, against this view of an "early date" for Revelation. I have always understood a better dating of about 90-95 AD.
It is a logical problem why God would appoint one apostle to "watch over" another one, as you rightly point out.
As to whether the churches could sink so low in such a short period of time, it is interesting to note that Paul references at the very least an apostasy that is almost complete in Asia (Asia Minor, home of most of the Revelation churches) in II Timothy 1:15. When one reads church history it is AMAZING how quickly the churches become moribund, powerless, dead and lifeless when God withdraws His Spirit, and conversely, when revival comes, one wonders at the amazing power of God to completely turn a church (and consequently a culture) on its head.
Irenaeus, as we have seen before, is clearly chiliast. It is important, however, to note two things as well:
1) Irenaeus himself acknowledges that the NON CHILIAST VIEW WAS PROMINENT AS WELL AS CHILIAST. I don't know whether it was to you or to someone else, but I have already mentioned how Irenaeus "argued" his chiliast position with other church members, indicating that the "a mil" view was definitely up and going strong, and that there was a split in the church view even at that early age.
2)Arguing a "historic pre mil" position is a radically different thing than arguing a dispensational pre mil position. Historical pre-mils are essentially covenant theologians who argue for a 1000 year reign of Christ, without all the gobbledegook of a secret rapture of the primarily "gentile" church, and setting up a revived Jewish theonomic state (i.e. "Israel"). You will find very little (actually none) support for the dispensationalist schema in Irenaeus, or Papias (no original writings exist). Irenaeus clearly teaches the church will go through persecution (the tribulation!) under the antichrist.
Is it possible that this is because the Non-Chiliasts rejected the Book of Revelation as Scripture, and only in the Book of Revelation is the 1000 years prophesied. It was after all the last book written and many refused to acknowledge it as part of the scriptures. Once it was accepted, the chiliasm therein had also to be accepted --- unless they were disciples of Origen's allegorical readings of scripture.