Saber war didnt have Welfare, but Halloween have ~

You can clear Halloween event without use any single Apple, this event quite easy from what i see cause of the new mobs mechanic, you can use Apple you get from the point reward when the best node come out soon ~

And make sure to buy the old event CE on the shop, we didnt know if its will appear againt or not next year, cause this is part of trilogy

You can always play halloween again 2 years later

You can also play saber wars 2 rerun 2 years from now

Finish one event first, and then move to next event. That's how I manage 2 accounts at events. My main finishing all stories and such, and then my alt comes after that. For SW II, you can also make "daily mission target", like the event last for 10 days, so everyday just make sure you finish atleast 10 missions.

When you have more than 1 account...

I think Halloween 4 is more important since you will get more welfare. There is nothing much you will miss in SWII except for some lores, gold fous, and tickets. I think you can try completing Halloween 4 first then try to rush SWII

Use gamepress to help u, it help you complete any event in less then 10 total game play hours

You can play both

I do both with no problem, especially with SWII walkthrough available

You can either do both or just play halloween 4. saber wars has a rerun in na later on so wont really miss out on anything but if you can do both just do both and manage your time

allocate 1 day for each. grind whole day jp and the other day on english server