Stacking's not that bad. When you're ult is on, it stacks for any tick that the tear can accommodate. The entire argument against the game being decided before it's stacked can always be levied against tear. If you follow just that logic, then tear is literally always the wrong choice on every champion, bar none. That is clearly not the case. However, it is situational. I'm not saying that this build is always appropriate for every game, it most certainly is not. But you have to read the game and teamcomps to be able to determine whether or not you need immediate power, or if scaling power is going to be more useful for you.
You can certainly swap out frozen heart for a zhonya's, it really depends on whether or not you're more concerned about flat instant burst, or sustained damage, as zhonya's is stronger against the former, and frozen against the latter. The idea in this build is that the seraph's shield (which will be massive with such a large mana pool) should be enough to survive burst, and you have enough healing and moderate resistances such that you can survive most follow up damage.
It's also worth noting just how cheap this build is, the total of the listed items is only 15.6k! Rod and archangel's are easily some of the most cost efficient items in the game, tank items are cheap, and Rylai's has crazy stats (though that'll change in the near future). Again, it's not a build for every game, but it is a viable option where you sacrifice early game power in order to have a strong late game build that completes faster than most builds of comparable strength. It's also a lot tankier than most builds, so it, again, is somewhat dependent on your teamcomp. This is for when you need a beefier frontline more than damage.