John
"There is a rather large leap from 'is' to 'is' must occur 'one at a time'"
Not so. It is all the same thing. It is an inherent feature of existence ('is'). For physical existence to occur, and then re-occur differently, which it does, and which is where we must start with science, that can only happen 'one at a time'. Because unless any given physically existent state in the sequence ceases, its successor cannot occur.
We do not "experience time as a series of occurrences/past to future". We experience a sequence of representations of presents with a duration delay when comparing when they each existed, with when they were each received. Physical reality is not "turning future into past", it comprises a sequence of presents. When any given present is superseded, it then constitutes a past. The future does not physically exist.
"The 'is' is the present. It doesn't go from one present to another present"
Not so. Obviously it must do, whatever is existent at any given point in time was the present at that point in time. There are only presents, that is why the word is used.
"The one present just has changing physical arrangements."
Not so. What are "changing physical arrangements" then? They are the physically existent state which occurred as at a point in time (a present). What you are alluding to there is whatever fundamentally comprises physical existence. But this is 'just' the 'stuff', it has a particular physically existent state as at any point in time.
"Now you are going to dispute all this, so take it one step at a time"
Yep, I have just done so! [But I would take this opportunity to acknowledge the way you, JCN, and others on occasions, keep the discussion going]. "If time is a series of distinct presents". But it is not. Time is nothing, it is a false concept. The sequence of presents involves change (which can be identified when comparing one with another) which has: a) substance (ie what changed) and b) frequency (ie how quickly it did so). Timing is a measuring system which calibrates the latter, by comparing the number of changes in one circumstance against the number that occurred in another, over the same duration.
"where do they come from and go to?"
The next one in the sequence is the previous one superseded (which involves its cessation-by definition). This is the point, nothing in terms of physically existent state, "comes from" or "goes to" anywhere, only is.
"What is this underlaying mechanism"
Now that is a question. What innate properties are causing this sequence of alteration, how do they work, etc, etc.
"Is it the 'fabric of spacetime,' with some extra-dimensional 'blocktime,'
No, both of these are flawed models of physical reality. Just stick with the above question. Whatever ultimately comprises reality (and there may be different types) and hence is the 'substance' of physical existence, must have properties which cause alteration, either of themselves, or when interrelating. And don't ask me what they are, because I have no idea!
"It's not that I'm doubting you, but just trying to examine all the questions which arise from your model"
As stated above, I very much appreciate your questions. Hopefully, in the same spirit that I expect people to operate in, I too provide relevant and reasoned responses.
Paul
PS: Murray tennis match is now underway