I think you are defining relationship story too narrowly. Relationship is not just about conflict between IC and MC, but about the relationship itself. This means that sustaining a relationship can be a problem as in ET's case. This is just a matter of storyencoding and has no bearing on storyform. It could've easily been encoded otherwise. Sometimes relationship solution breaks up the relationship, other times it strengthens the relationship. Sometimes solution is not introduced, other times it is. Sometimes breaking up relationships is a good thing, other times it's not. Author gets to decide all that.
I think what Chris meant to say was that the relationship was growing rather than dissolving. This is because we see Elliott and ET starting off not knowing each other, then ending their relationship with a strong friendship. In a dissolving story, ET and Elliot would've started off as friends and drifted away from each other as their friendship crumbles.
In ET, the RS is strained because of the outside pressure, making it harder to sustain their relationship. The fact that they were continuing their secret friendship was the problem. And their whole relationship was driven by each other wanting to do the right thing for the other. I think I can see bits of solution (temptation) sprinkled here and there (ie. ET getting drunk, Elliot telling about a spaceman), but not to its full effect.
ET and Elliott's relationship is not resolved (solution is not introduced) and that turns out to be a good thing; they are always for each other wherever they are although they might be separated. However, imagine ET not being able to leave the earth and government breathing down Elliott's neck. One possible solution would be to surrender ET to the government, thus resolving their relationship, which would've been a bad thing (or maybe good? -- depending on how the author frames the story) for Elliott and ET's friendship.
Also as for Catalyst and Inhibitor, it is just used to either speed up or slow down the story. If catalyst is introduced, story speeds up in its progression (for the better or worse) and inhibitor stalls the progression of the story (for the better or worse).
that's just my 2 cents.