In answer to the general question how to interpret slashes in this context, there is no universally-applicable answer.
OP says the Indian modifier can't meaningfully be applied to an Apostille (an internationally-recognised form of certification). I'm not an expert on such matters; I know only what I've just seen in that Wikipedia link.
The link makes it clear that an Apostille must be certified by a competent authority designated by the government [of a state which is party to the international agreement to recognise Apostilles]. I note that an Apostille issued in Liberia, for example, is not recognised in Belgium, Germany, or the United States.
The implication is that any given Apostille is certified in the name of some country, which I assume in OP's case would be either India or the US. Feasibly OP could get the certification done by an authority in some other country, but I imagine this could be difficult or impossible (I certainly wouldn't recommend trying to use a Liberian authority!).
Returning to the headline question, the "continuing rightward applicability" of modifiers in a list a slash-separated items isn't fixed by any rule of grammar. You must either use your own pre-existing knowledge of feasible combinations, or ask the author for clarification.