FOBS was banned under SALT II but is not currently banned by New START, note Russia's constant talk about a "South pole" capability on their new Sarmat. FOBS is also not considered to fall under the Outer Space Treaty. Any "South pole" attack will suffer accuracy issues which can be solved via the use of GPS, this is not a FOBS-specific problem.
The US has consistently interpreted FOBS of the actual LEO type as being banned by the Outer Space Treaty.
The accuracy issues tend to be worse for the full orbit types which has merit on the LUA-without-apocalypse front.
So Russia would be concerned about SLBMs but not by a FOBS attack on China?
The Russians don't yet have a satellite constellation that can consistently detect random ocean launches, their
radar coverage of the Pacific is worse, their ASW isn't great, the SLBMs have actual short time of flight capability
and the entire US SLBM warhead inventory will be capable of threatening their mobile/fixed arsenal.
A CONUS based launch is well within their ability to consistently detect and with their southern radar coverage
track as well.
Do note that such an attack could very well be aimed at Russia, all the warheads have to do is keep going a bit more before deorbiting.
The point is that it won't blindside them and doesn't attempt to exploit gaps in their early warning.
And per your own argument, their are fewer weapons required to attack China than Russia so in combination
with the above, a China-sized attack is much less likely to be construed as an attack on Russia.
MM3 is more likely to be used if it is under threat. Since China can not threaten MM3 there would be less need to use it against China.
Only a small portion of the US ICBM force was ever LUA.
China "today" cannot threaten MMIII but GBSD is supposed to be a 50 year system.
You say there are fewer tubes on the subs but the subs carry more than twice as much warheads as the MM3 force
And fewer subs.
And world + dog regards them as the first strike option.
Not what you want to be slinging around in a bad neighborhood if you don't have to.