I think he would be a good choice, but I accept he just does not see her that way.
I think it's up to him to choose who he wants, if anybody.

I've read that a few times already and am still not getting it...Molly?
I think it's meant to be taken that she's single and never been married.
Certainly in the UK, that is how most people would take it.

I'm not sure ... I accept that technically, "umarried" can mean widowed, but I've never heard it used that way.   It think it would be used more in the context of "unmarried mothers" (if we were still in the 70s!) "an unmarried couple", etc. 

I also think "alone" applies more to Molly than John, as John has Sherlock (and Rosie).  But I suppose you could say that he is more alone because of his recent bereavement. 

I'm not sure John is short enough - he's a little on the short side, but Sherlock thinks that the coffin is for a woman because it's so small. 

Taking it a bit further as well, I don't see it being much of a problem for John to say "I love you".  I suppose it might be difficult if he wasn't there in the room already, knowing what's going on, but if there had been a phone call instead, it wouldn't have been painful and exposing in the way it was for Molly.   I think the task was meant to be difficult for Sherlock (and Molly) in a way that it wouldn't be if the person had been John.  He is doing something very hurtful to somebody he cares about. 
 

For whatever reason, this thread escaped me back in the day. Didn't see it until now. 

Yeah, the description could fit John as well. And it was probably intentionally vague so that people would wonder who it was at first. But, yeah, it did turn out to be Molly.

I never thought the "I love you" was real either, as in Sherlock loving her romantically. But I do think he honestly cares about her a lot, maybe to the point of loving her as a friend. Which is, as I see it, why he smashed the coffin. Knowing how much he's hurt her before, and knowing tha the just hurt her and pushed her too far yet again, it made him angry, sad and frustrated. Sort of an accumulation. 

So I do agree with the blog about one thing - if he hadn't cared about Molly, he wouldn't have reacted like that. I just don't think he loves her romantically. (If that was the case, we would have had a Sherlock/Molly-ending to the series, or at least hinted at in the ending montage. All we saw was her visiting again with a smile, which to me implies that they've had a talk and sorted it out).

Good point.
We saw but did not observe!