Killing Tiamat inside a Forcecage

[Eldariel]

Yet you've not really proven anything and certainly not proved what you set out to.

You can say that but that doesn't really change anything. Feel free to run the test on your own if you want to use your rulings. All the relevant complaints have been addressed: the remaining stuff is at most something where a DM might rule one way or another (which is an endless swamp), and not impactful enough to sway the conclusion one way or another: Tiamat isn't worth CR 30 if it needs a favourable DM to fight a level 14 party.

Therefore we have shown that a level 14 PHB only no magic items party can take out Tiamat in her temple as per the module with resources to spare. This doesn't go for every party nor every run but they are heavy favourites to win [as evident in the resources left at the end of the fight], which indeed proves (even more so than her Forcecage weakness if ruled to 20' size) that Tiamat is way too weak for a CR30 even on the tactical scale.