In the US, is there any FAA Regulation that keeps all aircraft away from 18,000 feet MSL… Since theoretically, FL190 could be lower than 18,000 feet MSL, and create all kinds of confusion with keeping safe aircraft separation?
That's where the transition level comes into play. When climbing through 18,000 feet MSL (transition altitude), the altimeter will be set to the standard pressure of 29.92 in.Hg or 1013 hPa. However, when descending, the local QNH will not be set at the TA, but the transition level (TL). The transition level is dynamic and depends on the local air pressure.
Following a table on how to determine transition level from Wikipedia:
The switch to local QNH happens, when passing the determined TL.
The lowest flight level one can fly is TL + 500 ft but many countries also add a buffer to TA.
So that's how aircraft can fly at both TA and TL while still having sufficient vertical seperation. (Transition layer + buffer)
It's not the altitude that is adjusted, but the flight level. Aircraft are allowed to fly at 17,500 feet MSL. Per the rules provided in other answers, this would be VFR flight to the north or east. an IFR aircraft could be flying at FL180, and this would provide 500 feet of separation.
However, 14 CFR 91.121 determines the lowest usable flight level. If the altimeter setting is 29.92 or higher, FL180 provides at least 500 feet separation from the next VFR altitude, and at least 1000 feet from the next IFR altitude. However, if the altimeter setting is lower, then the lowest usable flight level is higher than FL180.
Current altimeter setting Lowest usable flight level 29.92 (or higher) 180 29.91 through 29.42 185 29.41 through 28.92 190 28.91 through 28.42 195 28.41 through 27.92 200 27.91 through 27.42 205 27.41 through 26.92 210