There would be no way to prove that they are part of the chain.
Yes - I considered this. I think it is not a problem, because it is very unlikely that someone can prepare 0.01*MaxBlocks different blocks (unchained, as you noted) which have accumulated difficulty more than 1% or so of total accumulated work spent on main chain for whole time of Bitcoin existence.
In the
High hash highway, a second pointer is added. Headers would have a link to the previous block and also an "up" link.
This allows you follow backwards from the leaf of the chain much faster, but still hit all the high blocks. However, it is a hard fork.
The 2nd link points to the nearest block with more leading zero bits than the previous block.
I should look closer to your proposal. For now I have one question:
Does "up" link points to future? If so - is it included in block hash or can it be falsified?