What does it mean to be an opening theoretician, and how does one achieve this status? Does it imply that one should not be an active tournament player? Apart from opening theoreticians are there middlegame and endgame theoreticians, too? Why are most theoreticians from eastern europe?
A theoretician is someone who has created a new opening, middlegame and / or endgame variation. You don't have to be a grandmaster to become one, and not all grandmasters are ones.
It may not apply if your variation is not tactical or is useless.
I don't think it matters where a theoretician comes from, as everyone has different chess experience, and whether you are an active tournament player or not.
People who have written about chess theory in general are chess theoreticians.
Its just like in mathematics when one doesn't need to be a university or college graduate student with a "Batchelor of Science in Mathematics" to be a " mathematician. " All that counts is " New Idea," a workable or applicable " mathematical ideas " Ideas that works, mathematically. Same goes apply to aspiring "chess theoretician."
What does it mean to be an opening theoretician, and how does one achieve this status?
Those who write about chess theory, who are often also eminent players, are referred to as "theorists" or "theoreticians".
So to be an opening theoretician you have to publish some opening theory. Being a famous player would help sales/credibility, but is no essential.
Apart from opening theoreticians are there middlegame and endgame theoreticians, too?
Absolutely! The same wikipedia article discusses chess theory across the phases of the game.
Why are most theoreticians from eastern Europe?
Based on an analysis of number of grandmasters per capita, more are located in eastern Europe, implying that chess is more popular/better promoted there. Therefore peripheral activities, such as publishing articles about theory, carving pieces for sale, making chess clocks etc. are more likely to thrive there.