well, I don't disagree that B-21 is a threat, but there are plenty of other major threats like nuclear submarines for example. F-47 in the future and even B-52s operating from Alaska and Australia.
given the pace of PLA"s transformation, I think that might be too much work. For example, I think just the revelations from 9/3 parade alone makes operating in the theater a lot harder for certain US assets. It remains to be seen how that works out.
heat only matters for IR stealth, RCS is independent of temperature. IR emissions are purely LOS and angular sizes of targets are very small compared to FOV.
If supersonic missiles cannot be LO, then supersonic fighters wouldn't be LO. Being LO doesn't have much relations with speed. Even hypersonic missiles will reflect radio waves the same way a subsonic missile would. Yes, faster missiles will have more air-drag and thus will generate more heat, but that is Heat detection rather than radar.