Could you possibly describe the issue more? Is it like a case of mediafire itself having issues or something, or is it something else like you can't get to the download link?

Could be many cases, could be internet/wifi connection cuts out during download, you don't have enough space on your device, don't have a way to read the .zip folders, etc.

It depends on the update. Some updates either add more stuff on the existing map or rehaul certain sections of the map, some may just be exclusively adding new expansions, some may be a combination of the two. Recently since Update 14 it's been a mix of expansion and updating/adding more stuff to the existing map.

We're unsure exactly how big the map is. It's possibly like 6k by 8k, maybe 5k by 7k, 9k by 12k, we're unsure as we haven't entirely taken the time to look for specifics with how we tend to not have a perfect square or rectangle and have updates that can drastically change the size of the map. It's decently big though.

For the borders, yes most of the map has large mountains on the borders that genuinely stop at the edge of them, plummeting off into normal Minecraft terrain or something that is not the actual map. For the south western section of the map it's harder to tell because it's ocean but the ocean does eventually end and such. We also have an overhead map pinned in our Discord server which shows the borders of the map very easily.

Well you won't be able to disable the spawners, as those will always work unless you set the difficulty to peaceful (which isn't advised obviously)

In terms of making it for modded mobs to spawn by the spawners you'd need to do those changes yourself via putting the modded mob spawn egg in the spawner or by finding a way to select an area with spawners and change them.

For the initial way of disabling so zombies can only spawn by spawners, you COULD turn on natural mob spawning, but that'll make it so ALL hostile mobs spawn and exist. Using a mod like Bad Mobs could be used but it's not the most recommended because you could break some sections of the map by not having mobs used spawn (like we use not just zombies, we use pillagers, skeletons, witches, spiders, etc.) that if blacklisted from spawning will make things unable to be continued, loot unable to be acquired, and generally just be confusing when a boss doesn't spawn.

While in the Bad Mobs config I do know that if you only disable their natural spawning and let them spawn via commandblocks and such that it'll work fine but I don't know how to do that so you'd need to investigate that yourself.

Update 18 is taking longer than we anticipated and we're unsure of when it releases. Could be in a few months, could be in a year, could be in 3 years. We're really just unsure, and all we ask is you be patient and just be hyped for any leaks/teasers that get posted and for when the update finally releases.