I've setup multiple domains on the same Protonmail account. You can create new addresses (@protonmail.ch/protonmail.com/pm.me/domain) with a paid account (iirc up to 5 total addresses with Protonmail Plus).
That's simply an option, because the devs recognize that not everything adheres well to a tiling workflow. It's not intended to be the primary mode of operation. I personally almost never use floating mode.
You can enable tabbed mode for a split (or the entire screen) in i3/Sway with a single keypress-- it produces the same effect as what you describe, just automated.
edit: You can also use something like rofi to fuzzy search for windows and quickly jump to them, instead of alt+tabbing.