I think the vast majority if not all kiruv ppl are well aware of yichus issues. Every case must be taken on an individual basis. If you actually learn halacha you might see the standards for accepting someone as a Jew are different than what you imagine. There’s no concept in halacha of “beyond a reasonable doubt”. We deal with chazaka, reiusa, edus, anan sahadei, etc. It’s a complex sugya and real, responsible poskim should be consulted. Just because we know there are a lot of people out there with questionable yichus doesn’t speak to the status of any given person that coming before us and in my opinion לענ”ד shouldn’t put a damper on kiruv efforts.