You're missing a very important first question: why are you learning Greek? That is, what is it in Greek that you intend to read? The answers to the other questions follow from that. See Greek Dialects: Where to Start
for some of the issues.
ISA wrote:I1. What is the 20% core of Greek grammar I should focus on?
This is the trickiest question of the bunch. There's a lot of Greek grammar. Textbooks tend to start with the easy stuff and end with the stuff people somehow imagine is more difficult. The unfortunate result is that many of the most common things come late in textbooks.
Clyde Pharr's Homeric Greek: A Book for Beginners
(available in PDF in the "Learn Ancient Greek" area of Textkit) has a long introduction, which includes a large chart tabulating the most common forms of the verb at least.
Master present and aorist participles. Master contraction. Master the -Î¼Î¹ verbs (they come late in textbooks, for no good reason â€” they're hugely common).
2. What is that 20% core of Greek vocabulary that I should focus on?
There are several tools available to answer this question. First is the Perseus vocabulary tool. It can generate lists of the most common words in any author, and will certainly let you pick the top 20%. There are several books focusing on particular periods and authors. For Homer there is Homeric Vocabularies: Greek and English Word List for the Study of Homer
; for Attic prose there's Classical Greek Prose: A Basic Vocabulary
3. What is that 20% core of Greek pronunciation that I should focus on?
This is meaningless. Focus on all of it, whatever pronunciation you choose. Greek phonology isn't that complex.