Interference or not.

heck yes... you got it right

IMO You got it right. I'd go a step further and ask if there was anything close to MC because this sounds like a pretty aggressive move on the part of R1.

Did you call the batter/runner out as well? You should have. And from your description of R1's actions, you may have had good cause to dump him for MC. HTBT for that.

I'm assuming this is a league with the FSPR. Such as FED.

I don't think it qualifies for an OBR game with no extra exceptions. B/c in OBR, I believe it is the responsibility of the defense to avoid the runner. Which is why there is no "peel out of the way" clause when the runner is in between bases still. And, as long as the runner can reach the base and did nothing intentional, then no INT. But, that is for straight OBR rules.

I could be wrong with this for OBR but I don't think this would qualify for INT. He went straight to the base and did nothing extra to intentionally interfere with the throw.

No MC, the kid just didn't slide, peel off or do anything to avoid the contact, he did go into the base, slightly on the inside, and ran chest to chest into the SS.

I didn't really take into account his intent, more what actually happened. I felt as if he interfered with the SS ability to make the play. Had he slid in hard and took the SS out, no call as long as the SS was close enough to the base to be taken out by a legal slide.
It wasn't a "bang bang" play he had time to get down, peel off, stop, duck or what ever but he didn't.