Download the 6.61 update manually, a friend had issues with the installer downloading it automatically also:
https://www.reddit.com/r/vitahacks/comments/71x9mf/adrenaline_installation_failure/dne4roo/
If you connect to your vita using FTP rather than USB you will see all the mount points/drives, (ux0, ur0 etc). Connecting with USB just shows the contents of ux0.
It sounds like you worked around this by copying the plugin to ux0 using usb, then moved it to ur0:tai using vitashell
Think of ux0, ur0 etc as drives on a vita, not folders. Just like C:, D: etc are drives on in windows.
ux0: is the entire memory card. ur0: and other locations listed in vitashell are stored on internal system memory, meaning ur0: is always available for the system to load things from no matter what microsd/vita mem card is inserted.
When using the sd2vita/gamesd plugin the microsd is mounted as the ux0: 'drive' instead of the vita memory card.
The sd2vita guides advise you to delete the ux0:tai folder as once you are using an sd2vita, any future guides/processes that advise you to configure ux0:tai/config.txt , you should instead configure ur0:tai/config.txt
Many guides aren't written specifically for sd2vita users so they reference the ux0:tai folder, just use ur0:tai instead.
At boot, taihen looks for ux0:tai/config.txt to know what plugins to load. If it cant be found it looks for ur0:tai/config.txt
As the microsd in the sd2vita isn't available until *after* the gamesd plugin has been loaded, the ux0:tai folder on the microsd cant be used (the system wont see it at boot and you cant load a plugin from a location that isn't available yet).
Taihen will however see the ux0:tai folder on your vita memory card if one exists. We dont want it to use that as we're going to mount the microsd as ux0:, not the vita memory card. So we need to delete the tai folder from the vita memory card.
Now there is no ux0:tai folder, taihen will at boot look to ur0:tai/config.txt, see the 'ur0:/tai.gamesd.sxprx' line and load that plugin (if it exists in ur0:tai) which will then mount the microsd as ux0, giving you the space you crave
If you later start using other plugins, ensure that the gamesd line is always the first line under the *KERNEL heading in ur0:tai/config.txt, eg:
https://pastebin.com/sZch2yC8