J0rrit,
Both DT and Sid have given some good advice. I can see you really like to understand things you are engaging with and this is why you have questions. The truths we are trying to uncover through meditation are, as the Buddha taught, hard to grasp and hard to comprehend. One of the main reasons for this is that all the intellectualising, verbal logic and resulting questions actually stand in the way of grasping these truths.
Our minds are highly conditioned: by education, societal demands and expectations. In particular they are highly conditioned to inhabit the logic/reasoning circuitry of the left brain which is where experience is woven into a comprehensible story of past, present, future and I, me and mine. This is where the internal dialogue or monologue that keeps us asking questions arises. In meditation we are reconditioning ourselves to dis-identify from this activity and move more into right brain activity which is where our connection with the present moment, the immediate reality and nowness of perception, connection with reality at its barest occurs.
I wouldn't worry about stages of Jhanas and all that carry-on. Just sit and practice, it you stop trying to get somewhere you'll arrive.
I may be wrong but my take on your situation is very similar to that above from DT. By trying to refine your understanding of where you will go the step after the next one intellectually you are stopping yourself from diving fully into the now of where you are. This learning you seek cones from experience, not words. It's not to say words are wrong or bad or that questioning is pointless, just that there is a time and a place for it and it is really important to let go of it and dive headfirst into the experience of repeated practice such that, as Sid says, you become the scientist exploring reality and the teacher teaching yourself.
I'll come to what I believe that means for your practice through exploring for a moment your direct question to me. You ask if you need to establish yourself in the Samadhi of the first Jhanna before proceeding to Vipassana. This shows a very common misunderstanding, that of Shamatha/Samadhi/Vipassana being practices. They are not, they are fruits of well grounded mindfulness practice. They are all part of the same cocktail. When I drink a fresh beetroot, apple and ginger juice for breakfast I can identify the individual tastes yet there is a harmony that is greater and different than cutting it into pieces.
Meditation is just like this. The fruits of meditation come to different people at different times and in different ways. Of course there is some/much commonality but even the Buddha taught different paths to different people. For some they were taught "dry insight" routes yet for most it was calming and concentration followed by development of insight (though whatever your practice you will grow all these fruits together, in harmony like the juice I brew in the morning).
All these questions running round in your head, these attempts to nail down the path intellectually, indicate to me that perhaps the wisest choice for you right now is to learn to concentrate your mindfulness on the breath and the body. Not working towards a goal or wondering if you're doing it right, because that just plays into the highly conditioned mind you have for seeking answers through reason.
It is experience of quiet, relaxed, insightful mind that seems to be what you need most right now. This experience will only be gained by dropping the expectations, dropping the attempt to intellectually comprehend the next steps on the ladder. You seem to need to experience the step you are on right now: through spending the time currently given to intellectualising to actual experience, physical, practical, moving beyond words.
For now it would seem that the emphasis in your practice would most profitably be directed towards developing shamatha and samadhi: as you gain deeper experience of your own being in the present moment and calm the mind the next step will become clearer to you. So the answer to your last question, in my opinion, is a very strong yes: keep your focus on the breath and the rest will become clearer. But seriously, for now, don't engage the mind in speculation about what the rest means or will be: ground yourself in the now through experience of the breath and let yourself distance from the questioning intellectual side for a while: it doesn't hold the answers you seek and is, at this point, actually a distraction from them. There will be a time and place to re-engage with it but by then you will have grown some different neurons and synaptic connections in your brain, and connected to reality differently, so the questions will, in all likelihood, also be different.
It's all one taste: shamatha, samadhi, jhanna, Vipassana, they're not discrete entities but part of a journey you undertake and only by undertaking the journey will you begin to understand the landscape you are walking in and the landmarks you will encounter. At the moment your experience seems very intellectualised so you have a somewhat solidified understanding of these things and ask questions to see if you are sitting on the rock you think you are sitting on. Yet asking questions isn't the best way to find that answer: it's by feeling the rock under your arse.
Kindly,
Matthew