Awesome questions, i understand your worry! Firstly, my one and only reason for mixing additives first
This is not the case with all brands... BUT Your bases (grow/bloom) are made of tiny molecular formed crystalline salts and because of this they can react if mixed in the wrong direction because of the dilution rate, If you put your base in first you are adding a salty solution, then say you add 'silica' The silica will react because of the way it has been 'precipitated' and this happens if you add this in concentration on top of your base. Now this may seem like nothing, but if you've noticed your liquid go cloudy after adding Silica it is because it is attaching itself to the other carbons rendering those attached inert. The molecules are simply to large for the plant to ingest/metabolize/breakdown. If we add Silica into the res first what happens? it dilutes instantaneously with no cloudiness. After adding other additives or whatever following your base, you will the notice how nicely it all blends in, and doesn't look like a morning hot chocolate
This happens because the Silica is already diluted and mixed into the water and not in such a concentrated form
This took some thinking a while ago, but my logic tells me this should be correct with what i've learnt about chemistry
It isn't WRONG by any means, but there's just a better way, thats all... Like Heisenburg on Breaking baD
The difference in the end, and the point of my babble is you won't have any premature salts binding and building up before they would normally. Try it if you like
The difference in ph is nil at first but a few days after the salt has attached to the roots and starting to play games with meters giving false in-accurate readings... again from personal experience even being fastidious with cleaning and buffering meters in the past.
I always push my plants VERY hard, by the 3rd week of grow i'm pushing the boundaries of full strength base nutes and 3/4 additives. My tap water is 6.8 and balances out great, and no i don't use a ppm meter either! This could make things more accurate, and i have a tri-mater somewhere round the place if i ever get into issues ( still in the box ) But from what i've learnt from a good Mentor is that Ph is more important than ppms, they both work off each other, and against each other, making them an outright pain to get a true perfect reading and result, I watch my leaf vigor and her smell which i feel is more than enough if not better that a ph/ppm reading. My system is very quick acting, and as soon as the nutrient starts flowing there is a safeguard window i like to call it where the plant adjusts to the mix like a dog getting comfy on a pillow
and within half an hour i'll know if she's spot on and i can goto bed. This system has worked out to be quite an accurate way to grow, and ease of cleanliness is awesome as the root zone is like a kid in a candy shop!! Sorry for the dodgy jokes, i'm so mellow right now i'm almost dreaming/sleeping/typing did that make sense? Perlite is far from soil, as the chemicals do not bind to it, so each flush you can be less doubtful about leaving behind salts.... Its harder to 'over-toxify' this way unless you dose it too hard, and forget to double check or something silly like that, its almost foolproof, and everything is used for a particular purpose.
The first question can be answered above
I would never feed a new clone full strength, always work from around half strength, and then follow up at brief intervals. Naturally the ph will rise towards that of tap water by about the half week mark. This is because the chelated minerals have been used up in the BASE food, time to add another top up of fresh water, and 1/3 of recomended dose and ween down till the end of the week wit top up with water if needed. ( i use a 45 litre tub )and at the moment she downs nearly 20 litres in the day given her every 3 hour feedings
Leaf is a good telltale sign, if leaves are droopy but warm, she's being over-watered ( hard to do with perlite but..)
A big plant will more than likely transition faster due to the Much larger amount of surface leaves that are gathering energy. Plus i flush extremely thorougly and add the flower nutrient ASAP
IF i missed anything let me know!
not that thats a bad thing right now, as long as i stay on top of it and clear things out it will be fine, the 2nd net is in now, i'm not expecting much stretch at all, and i'm giving her an early flush and some fresh nutes to kickstart her flowering
really looming forward to seeing how big this monster gets hey! Going to be an awesome ride 
Jk. I've been trying to get a feel for what my plant likes. I just recently tried to push my PPM's over 1000 and started to get leaf curl. I drained the res immediately and went with plain water. I've been slowly raising PPM's since. I'm in first week of flowering. Manufacture (Fox Farm) recommends (1190-1330) but I'm at (615). I'm scared to go over 900 now. Leaf curl is my boogie man.