I never know whether to keep discussion starting from a 'Hide Details' drop down in more drop downs or what
Here’s a breakdown:
this signals we’ve got a conditional coming.
“if someone is caused to drink” / “if you make someone swallow (the other one)”
“then the curse would be distributed to two people and get weaker, so…”
“… you wouldn’t have turned completely into a fish y’know”
Does that help? It’s definitely one of those phrases that’s really really easy to get tangled up in.
With 飲ませていれば like that (not 100% sure to be honest if it’s causative 飲ませる or のます or if there’s a difference in this form anyway),
が would be the person making someone else drink/swallow something (in this case, our protagonist)
に would be the person being made to drink/swallow something (in this case, anybody else)
を would be the thing being drunk/swallowed (in this case, the magic thing)
So 2/3s of those are implied from context, and the form of 飲む is doing a lot of work, which is the kind of fun thing you can do casually in Japanese but would be a lot harder to get away with most of the time in English.
I believe it would be her own volitional, but inside the とする. Here’s a reference that can explain it probably in better words than I can. (I should probably try to avoid words like volitional and causative since I’m never 100% sure I’m using them right)