Oh boy ok, this question has the potential to lead to a dissertation-long answer so I will try to be brief and suggest additional readings so you can continue your research on your own.

I'm a historian of science, medicine, and technology at the University of Oxford, and I specialize in Soviet science. My specific area is Soviet cosmonautics and astrophysics, but I do have some general knowledge about other areas of Soviet science.

To answer your first question, yes and no (the very best kind of historical answer, no?). I'll admit I'm not an expert on Lysenkoism specifically, so there may be more information out there I'm not aware of (and hopefully some Lysenkoism expert will come along and supplement my answer), but if Lysenkoism is anything like other branches of Soviet science (which I believe it is), then its rise and fall is complicated. On the one hand, Lysenkoism wasn't working, and once Stalin died, and even after Khrushchev fell from power, Soviet policies on science became a lot more open/flexible than they were before. Also, during Lysenkoism's popularity (after Stalin died that is; Khrushchev was less brutal on science/scientists), the ban didn't necessarily prevent work on genetics and experimental biology. Soviets were incredibly interested in the prolongment of life and other gerontological fields of research, and there was certainly no way to accomplish anything in those fields without a grasp on Mendelian genetics. So as long as they weren't running around boasting of the successes of traditional genetic theory over Lysenkoism (and as long as they themselves were useful), there wouldn't necessarily be a problem. And, as I'm sure you can imagine, the ban was lifted (surely not coincidentally) when the Space Race was at its height and the Soviet Union needed to make leaps and bounds in scientific progress. When scientific success became more important than ideology (after all, surely Mendel's Catholic priesthood made him a worthy target of Stalin's terror), the policies rapidly shifted.

To answer your second question, yes, there weren't just purges of geneticists and biologists. During Stalin's reign, there were many purges of scientists, including seemingly innocuous fields such as astrophysics (here is a fascinating-yet-sad article on that: Mccutcheon, Robert A., ‘The 1936-1937 Purge of Soviet Astronomers’). This absolutely had an impact on Soviet science and progress, and I think an argument could be made that these purges were a key aspect in the eventual failure of the Soviet social and political system.