Kostya, don't worry. Testers will come for sure. You should just update app description on GP with note about beta testing. I recommend you also post this note also on some famous Android site (XDA or Android reddit).
Let's say 1.4.0 is in production, versionCode 1400
I publish a 1.4.1 , its versionCode will probably need to be > 1400, so let's pick 1410
Now I want to release a critical update to 1.4.0.
I have to use a versionCode > 1400. Will it also need to be > 1410?
Let's say I pick 1411.
Will the beta users get updated from 1410 (1.4.1 beta) to 1411 (1.4.0 with a critical fix?), going backwards?
IMO, If your beta testing channel will have 1410 and you push critical bugfix update 1411 to release channel, testers will get (1411 > 1410), but they of course lose 1.4.1 beta (1410). So for testers, you should do 1.4.1 with a critical fix. But maybe I am completly wrong.. Never test it myself. But you can test with no risk, it is free, isn't it? So If it meant more work for you, you could ditch it.
BTW, how my app works for you, guys? I was quite happy this morning, when I took my phone and saw notification about 1.4.0.27
But I am not satisfied with code I used
BTW, I had forgotten to tell you that I added some new strings. You can find them on pastebin (see first post). Try update them (I only need strings after tag <!-- v1.1 -->).