You state that Validator nodes have no power and it is a myth that they have any impact on the network.
Sorry. dinofelis is correct. Non-mining nodes have essentially zero power of enforcement.
When faced with a block, a node has two possibilities. It can accept that block and forward it, or it can consider it an invalid block and not forward it. That is all. What it cannot do is prevent that block from getting to another miner that is perfectly happy to create another block on top of it. If miners are extending the chain, demonstrably accepting those blocks by building other blocks atop them, there is doodly-squat that non-mining nodes can do about it.
Can SegWit's transaction malleability fix be grafted onto BU?
Yes. BU plans a malleability fix. Later, after more pressing problems (e.g., the hard cap on the transaction throughput production quota) are fixed. Whether or not that fix ends up being SegWit is still a matter to be discussed. At this point in time, the other leading proposal would seem to be FlexTrans. But there is nothing endemic to SegWit that prevents it from being ported to BU.
BU just more or less ignores the fix.
Not ignore, exactly. Just that it is better development practice to not introduce several new major features at the same release.