"BA US members can only buy an upgrade from certain classes, and I think that M (OWE) is not allowed."
Indeed. It is only from full fare Y economy fares.
"BA upgrades for US members is only a single-class upgrade, so from WT you only get into WT+ not into CW"
Not exactly true. If travelling on a route without WT+ ( e.g. all routes to LGW ) then the upgrade is to Club World.
Dave