There is no indication that MS hoped to avoid this. They made this Dev program extremely easy to sign up to, and publicized it and made it unlimited.
And you accepted the agreement, did you? Even if it's extremely easy to sign up, it still has an agreement that you have to accept. You have your full right to not install this Preview Developer and keep waiting for Official Release. But then you accepted the term and installed it. Don't say as if Microsoft lured you or forced you to do so.
I don't say that IE11 in this preview is Beta or not. But to answer OP's statements:
1) Like most of people here, I can open PhoneArena.com fluidly at ease without any issue (L920)
2) Like most of people here, after resuming to IE, IE point me to the point that I left before (unless I wait 15' or so)
3) Please provide your popup website so we can test it together
But I see your problem, the problem is because of Tombstoning mechanism. A deactivated app will have to save "state" and turn to sleep after a specific time of deactivation. When user access to that app again, that app will load that "state", in this scenario it'll reload your previous URL. That's how the phone handles memories without "giving" too much to apps, also the reason of "bad" multitasking compare to Android (and better memory control. Don't tell me otherwise, I'm also using an Android and it'd lag like hell if I didn't close opened apps. If you're using a Windows PC with many softwares open and run at the same time you can see it in task manager).
I believe iOS also uses this Tombstoning to control app's memories. Their timespan is 10' (if my memory didn't deceive me)
P.S: Of course I don't know if they can store position of webpage into Tombstone state or not. But if it's existed in WP8, I believe there's a reason for them to remove it in WP8.1.