I find a several of assertions here that beg for proof and/or further explanation as to why you believe they are true.
1. "The Methodists presume of all people what many Presbyterians presume of their children ..." Isn't there a fundamental and radical difference between the two? The Methodists base their presumptions on an unbiblical doctrine of prevenient grace, whereas the Presbyterians base theirs on a biblical doctrine of covenant.
I suppose this is not the proper thread for me to explain why I regard both to be unbiblical.
Regardless, when you say "Isn't there a fundamental and radical difference between the two", that echos my statement: "There are obviously some important differences".
2. Are you implying that regeneration comes from the use of intermediate human means?
Of course God can work regeneration upon a human spirit whenever he chooses to. However, the only kind of which we are able to recognize the fruits, is that which evidences itself in reception to the gospel message -- which we all insist is delivered exclusively through human means.
I'm sure it is true that many rank and file Methodists, just like their Presbyterian counterparts, "have no answer"; but denominationally, again just like Presbyterians, their theologians present multiple answers, some of them conflicting with scripture and with each other.
Interestingly, Methodists sometimes make a similar argument from the Abrahamic Covenant to justify their paedobaptisms. They believe that a child enters into covenant with God.
For a discussion of infant baptism that mentions both "preventing (prevenient) grace" and the above mentioned covenantal spin from the context of the Wesleys' writings (i.e. worship directives by John, and hymns by Charles), see
American Methodist Worship by Karen Tucker, pp. 89ff.
4. Are the similarities only superficial and intriguing because you assume that covenant children and paedobaptism are somehow connected to prevenient grace?
In both cases, they presume a work of (some kind of) regeneration upon certain people, and they assume that it happens even though the Word of God has not been preached to these people via human means. It's interesting from a more strict evangelical perspective.
If no prophet has yet been sent to say "Dry Bones, hear the Word of the LORD", do we normally assume that the bones have joined together, gained flesh and sinews, and are standing up on their own*?
No. So it's something of a theologican curiosity to me that some denominations make such presumptions.
* To be fair, the Methodists differentiate between this spiritual quickening and "Regeneration" with a capital "R", which at least John Wesley restricted in normal circumstances to the administration of baptism (see reference above). But as a Calvinist, I must see their theology through glasses they'd prefer me not to look through, because to me a human spirit is either dead (unregenerate) or living (regenerate).
You have painted with a broad brush here and I'd be interested in where the finer lines and borders occur in what you are saying.
I'm afraid you must be disappointed with my explanation, as I doubt the parallel is nearly as interesting to you as to me.