First of all, legal schools represent the difference between scholars in Fiqh issues, not theological issues.
Normally, in Muslim majority countries, there is almost always a Mufti of the state, or a person in such position, who heads a religious ministry or so. This ministry will then help common people in their questions and Fiqh issues, and will then issue regular Fatwa publishing on common matters.
These Fatwas and guidelines are usually based on a certain Madhab that is followed by the majority of scholars in that particular country or state, and will define what Madhab is followed there. Even in Muslim minority places, there will be usually a community with distinguished scholars that define the general Madhab that is followed.
Now, is it required from a scholar to follow a Madhab? It is not required per se, but usually is, and that does not mean they cannot follow other Madhabs, or in some situations, voice their own opinions, when they reach certain level of knowledge, and that it is discussed with other peers. (source)
A layperson is considered not literate and quipped enough to voice their own opinions on matters, and is required to follow a scholar they trust of knowledge and religion. This is just like when a scholar of science is not recognized if they don't have enough knowledge, and when they don't follow an established methodology approved by other peers on the same field. (source)
So coming back to your points:
- Yes, almost always a layperson has or follow a main Mufti or a scholar. Or other scholars who follow the Madhab of their main scholar.
- Not really only from one, but usually is. Grand Muftis or Mufti of the state usually will define what is acceptable in that specified place, but people can still hear and look up to other scholars.
- As discussed earlier, yes they usuualy have a certain Madhab they follow, but it doesn't mean they can't follow other opinions or make Ijtihad to derive other answers for their own contexts.