What is the basis for a blanket allowance of halachik compromises by organizations involved in kiruv, if any?
The purpose of this question is to see if there were responsa that give blanket allowances regarding kiruv (as I've heard there are).
It appears that one of two things occurred when you wrote your question: either you are bothered or puzzled by the things kiruv organization do, or hopeful that they have some heter. I will say that the blanket heter that you are conceiving of does not exist. Though there can be found specific examples and certain "loopholes" to certain halakhic issues, the general reason for what kiruv organizations do are not solely based on those heterim. The general consensus among kiruv organizations is that in order to bring them closer certain compromises are made. It is a decision both on the part of the organization and on the part of the rabbis running it. Many a time do rabbis have to face such decisions. A classic one would be allowing a non-shomar shabbat congregant read from the torah or be the shliakh tsibbur. Classically halakha does not allow this (even if there MIGHT be way of reinterpreting halakha), but rabbis had to allow it in order to maintain their congregation, to be kind and fair, and to conform to the reality we exist in. all this said, there is a limit and a balance involved. Sometimes the balance can be lost, but the hope is that it will be achieved. If you want, I can find something that validates what I just said, but I have spoken to several kiruv organizations and heard many rabbis say this.
There really is NO heter to compromise on Halacha when doing Kiruv period.
Howerver when dealing with non-frum people there is a lot of thought that goes in to dealing with them and their Halachic issues.
For Example: lets say you have someone who lives far away from your Shul, and you know he is driving to Shul, you do not have to tell him not to drive (if you know he won't listen to you), unless he specifically asks you about it.
It is important to remember that we are trying to bring them closer to Torah, not Torah closer to them.
I am unaware of blanket heteirim but perhaps the discussion regarding inviting a non-orthodox person for shabbos knowing she will drive may shed some light here. Based on this post it would seem that:
An additional consideration is the concept of committing a minor sin in order to save someone from a more severe sin. We do not ordinarily permit such a violation, R. Akiva Eiger (Glosses to Yoreh De’ah 181) argues that lifnei iveir is an exception. You may violate lifnei iveir, you may cause someone else to sin, in order to prevent him from committing a more severe sin. This seems particularly relevant to a Shabbos invitation which may inspire the guest toward greater religious observance.
However, R. Ya’akov Ariel points out that in R. Akiva Eiger’s case, the other person is definitely saved from a greater sin. In our case, the invitee may not be religiously inspired and may have otherwise stayed home and done nothing. On the other hand, the guest will definitely violate Shabbos in the future. Perhaps we can permit lifnei iveir for the chance to prevent these definite violations in the future just like we violate Shabbos to save someone’s life so he can observe Shabbos in the future.