I'm not sure how textbooks classify に(は), but the important point is the structure Aに(は)Bがある, which broadly means "A has B". So, translating little by little:
Yet, [こういったAの対応] had [少なからず救われている部分]
So it should be obvious now that 部分 is part of the childish behavior, at least if you assume that the childish behavior is the same as [こういったAの対応]
Yet, this kind of reaction from A had a part which [少なからず救われている].
The subject of [少なからず救われている] isn't stated explicitly, but from context one would assume that it's "I" or "we" or something like that. So:
Yet, this kind of reaction from A had a side which we were saved by quite a bit.
Note that 部分 is the agent (not the subject) of the passive phrase (i.e. 部分に救われている), but Japanese relative clauses do not capture this relationship. The passive construction isn't really necessary in English, but is idiomatic in Japanese which tends to prefer animate subjects.
The translation 救う→save here isn't very good, the next line hints that the interpretation of this is that A's behavior, annoying as it is, somehow proves to have some benefit to the speaker.
My total translation, very free, but hopefully idiomatic, would be something like
Yet there was something quite relieving about this kind of reaction from A.
Firstly there seems to be some discussion about whether 少なからず modifies 救われている or あった, and I admit that my choice was maybe less likely than the alternative, but it doesn't really change the high-level structure of the sentence, so removing IMO irrelevant parts, we get down to
From here, if I understand correctly, the discussion goes on whether に is the passive-agent に or (my claim) に as part of a whole-part relationship construction. This, to me, seems to be the same question as identifying whether the sentence is parsed
[Aの対応]には[救われている部分]があった. (whole-part relationship)
For the first parse, I think it is fairly well established that relative clauses cannot have topics.
'First, you cannot use the topic marker (ha) "wa" in relative clauses, because topics and focuses are defined in a sentence, not a clause.'
Even if you argue that this is a contrastive は, you would get the approximate meaning "There was a part (where we) were saved by A's reaction (but not by other things)", which is very unlikely.
The second parse is the only one possible to me. Of course, 部分 should not be read literally as a "part", it's more like a "side" or an "aspect" of A's reaction.