"On" for "we" -- do you then refer back with plural?

[Jupiter Jones PI]

I wrote a story in French for one of those sites where people correct for one another.  I wanted to say, "It's your fault we're stuck here."  I wrote:

C'est ta faute qu'on est coincé ici.

The French person corrected it as:

C'est de ta faute si on est coincé ici.

He's a native speaker, so I'm sure his correction is right.  However, I'm wondering if I should have also added an S to "coincé" as in:

C'est de ta faute si on est coincés ici.

My thinking is "on" does not mean "one" here; it means "we," so maybe adverbs that refer back to it should be put into the plural as if I were using "nous."  Should I add this S or not? 

The other surprise was that the corrector changed my relative pronoun "que" to "si," which I thought meant "if."  The English sentence "It's your fault we're stuck here" implies "that" as in, "It's your fault that we're stuck here."  Shouldn't it therefore be "que" combined with "on" to make "qu'on" as I wrote it? 

[FrenchFries]

No, don't add the 's'. French people often use 'on' instead of 'nous' to refer to 'us.' Though 'on' can be translated to mean 'one', it can refer to 'us' in an informal setting. it's kind of like 'tu' vs. 'vous' I suppose.

With friends: On va manger là. (Note the conjugation of 'aller')

With your boss: Nous allons manger là. 

As for why it was corrected to 'si' I don't know. But I hope I could help.