Sry for TLDR.
A correction on my statement:
You have to demolish the entire complex/building [everything] at the location so that the purge attacks another base.
A correction for you:
You cannot build on someone else foundations, there is a distance limit how close you can build to others, thats in wiki described as Claim mechanic.
To your theory:
Yes it is almost certain, when someone build next to your first building in the purge region [can be just one foundation, can be a small house, the game count it as your original base in that purge area], his base will be hit too.
This is a common thing on PvP servers, when purge is targeting a player base, but between their spawn point and target player build is something that belongs to different person. In that case the purge will divert to it. Its kinda funny on overpopulated/overbuild servers.
Now to your another point, yes you are correct, the game will calculate attacks on base in region2. My apologies i was writing a reply as i read so a correction to my statement was not necessary, you understood well.
Just a note from me here to be absolutely clear for anyone who reads it at later date, i build a base, then a small bridge, then another, then final base in single purge area. I had to demolish even the bridges… So everyone should think really hard when they start building in individual purge areas where their main base will be, all other buildings after wards should be safe from purges [pain houses, bridges etc]
As to your final idea, yes it may be technically possible on server with few player buildings, to grief someone with purges. But that would take an incredible effort. So Devs doesnt have to adress it.
You would have to build close to his base your first foundation/building in that purge area, and be as a player in that purge area when the purge commence [as the purges takes into calculculation in which purge area the player is].
Also you never know at what time the purge commence once your purge meter is full. So you may end up spending up minutes up to hours in that area to make sure your purge spawns there.
A final note: What devs really need to is to correct a pathing for purges in volcano area, Purges there are still walking into lava and dying… But in my opinion thats something impossible to correct or extremely difficult to correct.