I very much agree with @ktjoseph Sometimes those of us who fancy ourselves “polyglots” have those big plans of “conquering the world” which, in our case, amounts to learning all the languages of this world or, at least, cast as big as a net as possible, maybe by learning a language from every important family in order to later “attack” other related ones and so on.
If you think in those terms, of course you need an “attack plan” to optimize the results.
I do respect anyone’s interests but in my experience that way of thinking ends up being unproductive. From a pure practical point of view you’re leaving lots of factors which are more important: What kind of content is available, will you be able/willing to engage with the culture whose language you’re learning? Can you meet speakers from those languages? Will you care to talk with them? Will you like the literature, etc.?
At the end of the day, motivation is key: when you learn languages as a means to an end, chances are you won’t keep motivated for very long. That’s especially true as you grow old and find yourself spending a limited amount of time in many competing endeavours.
Having said that, I’ll explain my own strategy about OP’s point 1, as Malay/Indonesian is my current main target language/s. I think this will also illustrate my main point:
The way I see it, in a case such as Malay/Indonesian I like to think of it as two varieties of the same language, as a “diasystem” to use a technical word. It is a very different situation from learning, say, French and Spanish. Malay and Indonesian are in fact closer than most Arabic “dialects”. I think the situation is more similar to Persian, in that there are many variations but mostly two main standards: Iranian and Afghanistan (debatedly also Tajik) and each of those also related to a main colloquial standard (Teherani/Kabuli) which in turns differs from the standard. Depending on your interests you may concentrate on one variety or another but to me it makes sense to treat everything as a single language, learn the most important differences and then read/listen whatever you find in any of the varieties. The main reason is my personal interest: I want to learn about the many cultures of maritime South Asia. Besides, that way I maximize the amount of material I can use and the chances of finding something interesting. For example, there seems to bee way more material in Indonesian than in Malay but sometimes I find something very interesting in Malay. At the same time., content in Indonesian can be in the formal register or in Bahasa Gaul or in a mixture. At the same time, Malay can also be formal or coloquial and the differences are far from trivial: my goal is to understand everything and in fact I find that some varieties reinforce others. For example in Indonesian the most usual way to say “want” is “mau” and in Malay “hendak” (colloquially usually abbreviated to “nak”). However both languages have the equivalent word of the other (mahu and hendak) so it pays to be exposed to both variants. In general the main part by far of the vocabulary (plus the whole of the grammar) is useful for both languages.
So, as passive understanding goes, I learn all. At the same time I only consciously activate a variety at a time, and I choose that based on my interest. I may go to Bali so Indonesian is clearly my choice here. I strive to speak in a medium register with some aspects of bahasa gaul but not too many because I think this is what would sound more natural in actual conversation with natives, which is what I’m interested in.
This is my approach, others would do it differently but notice that my rationale is based on my goals and interests… In particular interest in the culture and in traveling. The question of who would understand better the other language and so on does not play any role in my decision. I don’t even think it makes any sense to even ask the question because it depends mostly on individual capacity and attitude. I’m sure that a Malayisan person can understand you if you speak clearly in standard Indonesian and if that Malysian persanon wants to understand you and makes an effort. Ditto for the opposite cas
If you’re so interested in mutual intelligibility, I would advise you to watch videos in which people interact in different languages. There are quite a few. For example:
Brazilian Portuguese | Can Spanish and Italian speakers understand it? - YouTube [Portuguese/Italian/Spanish, many other such experiences in that channel]
Malaysia vs Indonesia: Bahasa Challenge #WeAreBackAgain - YouTube [Malaysian/Indonesian]
There are many others and they often come to different conclusions. For example the above video shows two persons mostly communicating fluently using their respective languages and having fun discovering differences. However, it’s not difficult to find the opposite view. I can’t find right now another video of a Malaysian person complaining about his difficulties when he visited Jakarta. This goes on to prove the importance of attitude for comprehension.