I'm planning on providing a ton of options for facing the bbeg in my campaign, and I need to know about planar travel.

I need some questions answered. The reasoning for them is explained afterward because the backstory is so long-winded.

1: What is the deal with cosmology in dnd? I'm hosting 5e, and I keep seeing references to spellplague, destruction of ethereal plane (why would you get rid of something so cool?), and mixing together of elemental planes. (weren't they already mixed in the wheel cosmology?) Is this all 4e stuff? Wasn't 4e like the red-headed stepchild, like the dbz gt of d&d? Is 4e chronologically after 5e? (I'm a very new dm)

2: What assured means are there of traveling to other planes? Off the top of my head, I can think of The Isle of Dread's storm, hydrothermal vents, and obscure magic items.

3: How do aboleths travel between planes?

4: How could the gods be sealed out of the material plane?

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Okay, let me provide some backstory: in my campaign, the world's aboleths are exceptionally close to reclaiming the material plane. They effortlessly control bacteria, viruses, and all manner of microscopic organisms with their psionic abilities, allowing them to manipulate disease, ph levels, water flow, and possibly even exo/endothermic reactions. They see the material plane as neglected and in need of repair, and sapient lifeforms as an infection of sorts.

They modify their plans depending on the times, inciting war when necessary, controlling the movement of civilizations by deciding what areas have clean waters, and which don't, and sparing those that present themselves as a non-threat (such as kuotoa, laboring/edible slaves, and those who are incapable of presenting a threat). Regardless of how they cull the populations of free thinking, sapient beings, they intend to finish off the problem by releasing an incredible number of water elementals & weirds that they can control by way of dominated humanoids that control them. Essentially, the elementals are weaved into a vicious, sentient storm that desperately wants to return to its home plane, and cannot do so until the magic wears off, or until they accomplish their task of eradication. The aboleths also plan to lure the celestials into the material plan, trap them there, be killed by the celestials, and have their offspring mind control them once they've been converted into titans by the material plane (which does happen in my variation of the world, as per [this thread on rpg.net] (http://forum.rpg.net/showthread.php?744712-Aboleths-Threat-to-the-Gods-or-Delusional-Pests).)

The aboleths have been doing this for ages, implying that they've not yet managed to fully eradicate the humanoids. Each culture seeks a means to escape the storm, and leaves behind a record of their methodology and plans. One culture converted itself into water elementals, allowing themselves to be absorbed into the plane of water. Another sealed itself in a subterranean lead bunker where they eventually starved. Many have ousted themselves to other planes, either dying of exposure or evolving into new races. Each one also leaves behind various plans for stopping the storm without sacrificing their world or freedom, but none has succeeded.

The players will have a list of theoretical solutions and resources to aid them in their quest. In particular:

How to locate the aboleth city, regardless of continental drift.

A list of all discovered means to escape into other planes, and what will be needed to survive in those planes. (Ex: the storm over the isle of dread)

The last known location of the tarrasque and the kraken with a warning that they are just as capable of killing your civilization.

How to locate a tagged leviathan and/or dragon turtle, provided that they still live, with a warning that they will likely die when they are used on the aboleth.

Schematics for an apparatus of kwalish (the size of a small house) fit for travel through both the deep ocean and the elemental storm (so that players need not travel between cities via the underdark), as well as experimental lightningrods, armor plating, and advanced swimming equipment for it.

Recipes for powerful explosives that adhere to structures and function underwater (hope is that they'll cause enough damage to the aboleth city that it will take years for them to resume their plans)

Recipes for soluble chemicals that function like tear gas underwater (but not in high enough concentration to subdue an aboleth).

Treatments for diseases found around aboleths.

A hypothetical means to temporarily seal the gods out of the material plane, forcing the aboleths to postpone their plans (giving civilization time to step up their technology).

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5: Ps, how do you, as a dm, handle situations in which the players want to acquire a specific magic item? As you can tell, a cloak of the manta ray would be invaluable in these missions of mine. It seems awfully uncreative to say "oh, well it just so happens that there's a mission available with the item you want at stake!" or to say "your reward for saving the city is... schematics for creating the item you wanted that we just conveniently had!"

I could always provide them with the items they need before they need them, but I want the situation to have a satisfying feeling. For example, in skyrim you might need to get a good supply of souls for your black star, but being a dual wielder, you wouldn't want to waste a valuable enchantment on your main weapons on soul trapping, or constantly open your favorites menu to switch between soul trap and your sword, so you would have to find and disenchant a dagger of soul snaring, and from there, enchant a bow to have 1-second soul trapping.

EDIT: Couldn't I basically just make up my own cosmology? Say "feywild and shadowfell are awesome, I'm keeping those. Negative and positive planes suck. I don't want that" etc.