yeah beav your deff on track for sure i think..this early it wouldnt make any sense that it would be anything but a salt build up. but its hard to tell 100% using a water test for ph. there are to man variables that will bounce the ph up and down its never going to accurate. depends how much your plants are eating too becasue if your run of contains a high volume of nutes then im fairly sure your ph will be higher. you must get a ph meter and an ec meter. there is a brand called ezitools and they are only 35$ each. but like i said earlier you must water your whole medium otherwise your going to get pockets through it soaking up more or less depending on where you have watered....if you flush you could also run a very very mild nute solution with it but i would opt for just pure phed water since your plant is so young. as for stretch it completly depends on the exact % of indica to sativa ratio. and you can top your plant when it has about 4-5 nodes which will normally make your plant about 20cm tall by time you do this. you can veg for as long as you want...if its a dom indica then you can expect 2x stretch in flowering, but it all depends how your other conditions are. for awesome results everything has to be on target, air exchange, light distance, temp+humidty, feeding strength. like i said if you can get the basics right you will be on your way