D&D, and by extension Pathfinder, is designed around characters who go places other creatures don't want them to (dungeons) and haul valuable items (treasure) back out. Inaccessible locales, hostile planes, powerful protective spells, dangerous monsters - all are designed to be challenging but surmountable obstacles. The game is specifically designed for this; It is hard to the point of impossible to build a truly impregnable vault/trap/fortress because that would spoil at least some of the fun.
As for foiling the gods... Well, you're unlikely to succeed. Most campaigns don't have any rules that define what a god can do, because the assumption is that they're so much more powerful than player characters that it doesn't matter. There's presumably some reason why this or that god doesn't simply remake the entire multiverse into his or her ideal (Certain settings have suggested some kind of delicate balance of power or mutual "we'll kill you if you move first" agreement that all gods feel bound by, or that there's some cost involved in divine intervention that makes acting through mortal agents the best option, or even that all gods generally like the world the way it is and prefer subtle influence when it comes to making changes because there's less risk of messing everything up that way), but the important thing is that what a god is capable of is limited only by what the GM decides the god does at any given moment. Unless the GM decides that the gods do have limits - and if you're the GM in this case, be careful not to make them too easy for PCs and NPCs to manipulate - you're unlikely to be able to stop them from retrieving anything.
GM fiat can solve the issue, of course, in a "It was cast into the vortex of Mec'ril, which was subject to that ancient and unalterable decree that none, not even the gods, may ever hold knowledge of its contents, such that those who enter forever lose all conciousness and thought even of themselves" kind of way... But you are unlikely to find anything in the rules-as-written that will help. Well, maybe an artefact, but those are only placed in the game by GM fiat anyway.
No, you're better served by making the thing you want to hide extremely hard to find or difficult to achieve for less-than-very-powerful characters. After all, powerful characters are rare, and very few of them will have knowledge of or interest in retrieving whatever it is that you're trying to get rid of.
Yes.
The ability to summon the chest is part of the effect of the spell and not of the casting.
While the chest remains on the Ethereal Plane, you can use an action
and touch the replica to recall the chest.
Since you do not have to cast the spell, whether it is prepared is irrelevant.
Best Answer
The most "potent" effect prevails
Under the rules for Combining Magical Effects, two effects with the same name don't stack. QED.
The only tricky part is the DM has to decide which is the most "potent". If one caster used a higher-level spell slot, that would be the most potent.; otherwise, the "most recent effect applies".