What is the relation and history of 'si' and 'sic'?

Question

Lewis and Short tell me that sic comes from si by adding the particle -ce. I can understand sice wearing down to sic, but I do not quite understand how I am supposed to understand the meanings of the words.

The meanings of si ("if") and sic ("so") seem pretty far. If I am interpreting those dictionary entries correctly, si is a form (locative, ablative, or something) of an extinct pronoun related to the Greek articles starting with h-. Forms of pronouns fossilizing into adverbs or conjunctions or particles makes sense to me, but I am not sure if I understand things correctly.

Here are explicit questions whose answers would help wrap my head around the matter:

1. Do si and sic come from the same form of the same pronoun (one with added -c(e)) or is the similarity coincidental?
2. What did this form of the pronoun mean? I would like to make sense of how the meaning evolved to both si and sic of classical Latin. (If they come from different forms of the same pronoun, what did they mean?)
3. Does -ce have an effect on meaning other than emphasis?
4. How did si and sic get so different classical meanings if they come from the same source?

Feel free to answer any subset of the questions you like. I can split the rest into a follow-up question once I know which way to continue.

Answer 1

Here is de Vaan's entry for both forms:

By this account (which I think is uncontroversial), both forms come from the locative singular of the demonstrative *so- "this", with an added enclitic *-ke in the case of sīc. This enclitic is the same form that shows up as -c, -ce in demonstratives like hi-c, huius-ce, etc., as well as some other words like nun-c, ce-do; it seems to have had (redundantly in many of its uses) a general proximal deictic meaning such as "this, here, now".

The semantic starting point is "in this (case)". For sīc, the shift from "in this case" to "in this way, thus" seems easily understandable. For sī, the grammaticalization to a conditional presumably took place in contexts like "In this case, namely X, Y will happen" -> "If X, Y".