What does this sentence including もとで mean?

Question

しかし、あの川のことがもとでじっさい兵太郎君は病気になったのなら、兵太郎君がそれをだまっているはずはない。

Short summary of context: Something happened with 兵太郎君 (heitaroukun) in the river, and his friends were there. The speaker is one of the friends, 久助君 (kyuusukekun). 兵太郎君 (heitaroukun) didn't show up in school ever since. Now the speaker asks himself why that is, or rather, if they (kyusuke&co) are the reason, Heitarou isn't showing up anymore.

I get the vibe of the above sentence somehow, which in my mind looks like this in English:

But, if in fact Heitarou got sick because of the river-incident, there wouldn't be any reason for him to shut up about this.

Question: How can I parse もとでじっさい in that sentence? Did I understand the sentence correctly?

What I researched on もとで:

下で: From a german/japanese dictionary: under, by, in. (I don't see the sense here if that would be it) 元で：I didn't find 元 with で, but I guess combining them the meaning becomes In fact(?). But then why is there じっさい behind it?

Answer 1

This もとで is 元で, which is a common phrase meaning due to, because of, etc. 元 literally means origin or cause. You can google "が元で" (don't forget the quotes) to see tons of real examples.

実際【じっさい】 is another adverb which just happens to be there (meaning actually, in reality, of course).