Some years ago I got a few interstem (different combo)trees from cummins. I was asking the same kinds of questions here is what Jim cummins sent back:
"We strongly recommend that trees on G.11/MM.111 interstems be planted
with half the interstem G.11 exposed above permanent soil level, half
below soil. This permits the G.11 to take root and the tree to
develop a dual root system. The precocity and productivity induced by
G.11 as a rootstock should be slightly diminished. The MM.111 will
provide solid anchorage; staking will not be necessary for tree
support (although we do recommend temporary support to facilitate
building canopy).
We expect ultimate tree size to be about that attained with Malling 26
– without the problems of fire blight, crown rot, and burrknots.
//Jim"
Some folks back then on gardenweb thought perhaps he was trying to avoid burr knot issues by burying the lower graft.
Hope thins helps some