Our mosque is only one in our Russian town. It was built 100 years ago. We've tested and found that Mihrab direction differs from true Qiblah in about 30 degrees! We spoke about it to our Imam and then he said to everybody that someone wants to cause Fitnah and change the Qiblah, but "we've prayed in that direction for 100 years and we'll continue". Now we pray behind him in the old direction. But I think it's not ok. What to do? The most of people think like the Imam.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is agreement among Muslim scholars that a worshipper has to turn his/her face towards The Sacred House at Makkah while performing prayer. In other words, turning the face in the direction of the Sacred House is a condition for prayers to be valid. Here are two exceptions: (1) in case of inability to do so, (2) one who is performing optional prayers while mounted is not obliged to do so.
Furthermore, it is obligatory, according to all Muslim scholars, for one who sees the Sacred House to turn his/her face to the House itself while praying.
But, if a worshipper does not see the House, he/she has to turn his/her face to the direction of the House, not the House itself. This is the opinion of the majority of Fuqaha Abu Hanifa , Malik and Ahmad are from among them.
The above-stated point of view is supported by Allah's statement in the Qur'an. Allah Says (interpretation of meaning): {…turn your face in the direction of Al-Masjid-al-Harâm (at Makkah), …} [2:150].
In the same vein, the Prophet (Sallallahu Alaihi wa Sallam) said: "al-Qiblah is between the East and West" [ al-Tirmizi and Ibn Majah from Abu Hurairah ].
The above-Hadith fixed al-Qiblah for people of al-Madinah al-Munawarah and those who are in the same direction and area.
As for other countries, their Qiblah will be between the south and north and so on, because if we say that they all have to face the House itself, this will constitute a hardship to them.
Allah Says (interpretation of meaning): {…and has not laid upon you in religion any hardship, …} [22:78].
There is also an agreement among Muslim scholars that a long row of worshippers in congregation is wider than the width of the House. This means that the worshipper who is at the end of the row will not be facing the Ka'ba itself, but its direction, and his/her prayer is valid without any difference among scholars.
In the light of the above explanations, we say that a worshipper who cannot see the Ka'ba is not obliged while praying to face the al-Ka'ba itself, but it is sufficient for him to turn to its direction.
Moreover, performing prayer with a difference of about 30-degrees from the true direction of al-Ka'ba makes no problem, although it is better to try to turn to the Ka'ba itself if possible. If changing the present direction you mentioned in your question causes a Fitnah, then there is not blame on you to remain as you are.
But, if one performs a prayer while the al-Qiblah is on his right or left, then his prayer is invalid.
At the same time, if one performs a prayer with a 45-degree change in the true direction, then his prayer is invalid.
Allah knows best.