IMO: Mix the concentrate. Shake well. Very well and mix you bottles from there. You could add the PG, but then you limit yourself in the future should you desire to change your ratios. I keep a premixed 80/20 bottle that I add following flavoring/nic. Not exactly precise; but I'm only mixing for myself. You could do the math based on your needs and this recipe and make a PG/VG mix, potentially with the Nic added for ease of doing 50+ bottles.

Either way I think your ideas are sound. I would personally do just flavors and a separate bottle of PG/VG.

Edit: as for the magnetic stirrer, I have no clue. I'm cheap and just shake the hell out of everything.

I usually mix for my wife, a few co-workers and myself. I usually just do a few bottles with 200ml and then shake it up and sit for a day. Then I shake and split up.

I usually just make a flavor base first, then just add nic vg and pg for each bottle, then concentrate and off to the next one.

Out of curiosity what magnetic stirrer do you use?

The best would of course be to mix the whole batch up all at once, then split into bottles (no need to shake each bottle, no need to measure). If that isn't possible, then mix the whole batch of flavors up, and then mix with PG/VG/nic in smaller portions.

Since it's not a business, you shouldn't have to concern yourself too much about steeping, just let your friends know which bottles need to sit for longer. If you want to speed up the steeping, you can make the flavor base ahead of time. As for flavors homogenizing with PG/VG, it's probably done by the time they receive their bottles even without the stirring.

mix flavoring in a 120ml and dispense into 30 ml bottles. top with nic and fill with vg. done

Uhm mix each recipe in 240mls (I get mine from hobby lobby), you should get you 48

As you're mixing for friends you don't need to be mega precise with bottle measurements you aren't a business. Normally me and my friends decide a recipe we are gonna try and I mix up either 500ml or a litre then pour straight into 100ml bottles. If your initial base is right then the bottles should fill evenly anyway. I use 100ml bottles with wider tops though so pouring is easier for me.

whats the fastest way to dispense my 1500ml jar into 50, 30ml bottles? i have a 100ml syringe but is too messy to use efficiently.

Not really answering your question but, who the hell vapes 1,5 liter a month?? Is it just me? is that a normal amount of juice to vape?

I have often thought of rigging a cheap back massager to an egg carton or something like a paint shaker

Ball each of the bottles up in a sock and throw them in the dryer with a bunch of towels