Continuing my experiments with 5.6 InnoDB online DDL, a bug which I've opened, and another which I commented on were quickly answered and explained by the Oracle/MySQL team.
On both accounts I'm happy to acknowledge the issue is resolved; in both cases I failed to produce a real bug scenario. Good lesson. Kudos for quick and informative responses!
What's left of my experiment, then? Still a lot to check.
I am mainly still confused with which operations exactly can use LOCK=NONE (allowing for updated to table while ALTERing). So far I am only able to produce ALTERs with LOCK=SHARED, meaning table is readable, but cannot be updated.
I will want to test speeds. I've so far been content with slow response times for queries over altered tables. How well will that endure under heavy load?