If I have a new WDW non-expiring PH and use it for the first three times at DLR parks, can I still use the balance of this ticket towards the renewal of my WDW AP next May?
You might consider asking this over on the WDW theme park board because Cheshire Figment, the ticket guru, might be able to answer it from a WDW point-of-view. I'm not sure the DLR angle would help because most of the DLR's passes expire (even if not used) and also have to be finished within 14-day of first use.
[Edit: Saw you already asked on the WDW board...sorry didn't see that until after I posted here.]
No, it doesn't change it. At DLR, each day you're turning one day on the WDW ticket into a one day DLR ticket, so there's nothing happening on the WDW ticket except that days are being taken off of it.