You seem to be again fundamentally misunderstanding what it means to run incompatible versions of software. It doesn't matter what you think rational miners will do. Once the 750 of 1000 blocks are found, there is no going back. It only takes a modicum of hashing power to start publishing blocks that are incompatible with 1MB nodes. This is not about "pushing Blockstream to get their act together." It's about avoiding the risk of breaking bitcoin forever.
and that "modicum of hashing power to start publishing blocks that are incompatible with 1MB nodes" wont be accepted by 1mb blockers and 1mb blockers wont stale their attempts. they would carry on hashing their own blocks and make blocks a few seconds later.. eventually even if it takes several blocks when 1mb gain height, they cause the C big blocks to orphan off..
miners wont risk it at 70%.. yes the setting will be active but miners wont push for more than 1mb at such low levels as the orphan risk is still apparent.
they would wait for a higher number.. and just treat the 2mb setting as an unused buffer for the future. when they are comfortable,
and when that time comes (im guessing 90%). then and only then would the small miners not be able to catch up to cause orphans and the small miners should upgrade or be left behind.
which they should have done earlier as they had enough warning