Okay, I can explain a bit. Yes, your MMR can fall below the average for B5. When this happens, your wins will give you little LP while losses will incur heavy lp losses. You obviously can't fall below B5, but your mmr will be below it.
Normals and ranked have two different sets of mmr. So your normal games do not affect your ranked mmr and vice versa.
The only way to fix it is to start winning more ranked games. Each win will increase your mmr, consecutive wins in a row increase it even further than just win / losing. I think it is after 5 or 6 wins in a row, the system tries to match you against higher ratted opponents because it is trying to find where you belong.
Hope this helps.
Edit: Forgot to add, MMR is used to try to find out where you belong. Sometimes you can lose games at 0 LP in a division but not get demoted. Your LP doesn't take a hit for that, but your MMR does. Also, for the specific question you have, if you are at b5 and keep losing, your MMR will keep dropping. You could win quite a few games and get back up to B4, but if you lost enough games where your mmr is still lower than that, the system is going to try to put you bacl to B5 u less you keep winning and fix your mmr.
