[Aiwendil]

Raynor wrote, quoting Myths Transformed:

But, if we dare to attempt to enter the mind of the Elder King, assigning motives and finding faults, there are things to remember before we deliver a judgement. Manwe was the spirit of greatest wisdom and prudence in Arda. He is represented as having had the greatest knowledge of the Music, as a whole, possessed by any one finite mind; and he alone of all persons or minds in that time is represented as having the power of direct recourse to and communication with Eru.

Certainly Manwe (and all the Valar) had wisdom and knowledge far beyond that granted to mortals. Does that mean that all their actions and decisions must be beyond reproach? Certainly not. Tolkien himself, in the "Notes on motives in the Silmarillion" text that you quote (MT VII), seems ambivalent on the matter of censuring the Valar. He says a little later in the text:

The last effort of this sort made by the Valar was the raising up of the Pelori - but this was not a good act: it came near to countering Morgoth in his own way - apart from the element of selfishness in its object of preserving Aman as a blissful region to live in.

In MT VI he also censures Manwe:

Manwe must be shown to have his own inherent fault (though not sin): he has become engrossed (partly out of sheer fear of Melkor, partly out of desire to control him) in amendment, healing, re-ordering - even 'keeping the status quo' - to the loss of all creative power and even to weakness in dealing with difficult and perilous situations.

So we see that Tolkien did consider the actions of the Valar capable of being judged, and indeed that their 'selfish' isolationism and maintenance of the status quo may have been mistakes.

I don't think that the bliss of Aman can be extended; as explained in the Akallabeth, this bliss stems from the very presence of the Valar; this extension would have meant, for one thing, that at least some of them would have to be in constant travel from Aman to M-E, or that they should be sundered from their kin for various periods of time. Even more importantly, their continuous presence on M-E would have been a challenge to Melkor's domain which he could not have ignored

I was talking about the period after the Battle of the Powers, when Melkor was a captive in Mandos. With Melkor defeated, it seems to me that the Valar could have taken up again the governance of all of Arda. Why not return to Middle-earth, or at least go among the Elves and teach them, rather than sundering them? I'm not saying that what the Valar did was necessarily wrong or without reason, but I do think that it's a valid question.

As for whether the bliss of Aman could have been extended over all of Arda, we can only make uninformed guesses. However, it is worth remembering the Second Prophecy of Mandos in this connection (from QS in HoMe V):

Then Feanor shall take the Three Jewels to Yavanna Palurien; and she will break them and with their fire rekindle the Two Trees, and a great light shall come forth. And the Mountains of Valinor shall be levelled, so that the Light shall go out over all the world.

So at least at one point (circa 1937) this was considered a possibility, if only in a kind of Messianic sense.