Taking your questions in turn:
"I assume the mandrel in the Lee die is a bit larger diameter than the RCBS mandrel."
Not really but that's the effect the Collet sizer has. You can mike it and will likely find the Lee mandrel to be smaller than convenitonal expander buttons but case spring back after sizeing leaves the neck inside diameters a bit larger than "normal."
"Is that the difference that allows low effort seating"
Yes.
"and does the increased neck diameter pose any problem? "
No, not really.
"Lee suggests a light touch with a fine abrasive to reduce mandrel diameter - has this approach been successful with Lee collet users?"
Such "grinding down" of the mandrell can be done and I've done a few myself, but later wished I hadn't. If the case neck "tension" is too low it's more likely due to excessive brass hardening and that can/should be fixed by annealing not dicking with the mandrel.
"Another assumption is that the Lee die reduces case neck runout..."
That's true. They are, IMHO, the best necks dies for factory rifles on the market, at any price.
"...to the point of greater alignment of the case within the seating die resulting in easier seating of the bullet."
The moderately light bullet tension is a BIG part of the easier seating, it's really not due to better "alignment" of the sized cases in the seater, as such. The high bullet tension/seating force that appeals to some reloaders actually insures a good deal of runout during seating in most dies. I've found best runout, and accuracy, with necks .001" to no more than twice that under bullet diameter.
"I use an RCBS seating die."
Well, they're better than nothing.
Forster/Redding BR/Comp seaters are significantly better - statistically anyway.