Map makers can get the portal block by simply writing /give <playername> 90
Yes, but normal portal blocks stop existing if there's a block update and they're not in the proper obsidian frame. Also they're destroyed by explosions.
This would be a second portal block, identical in appearance, that can be placed anywhere and isn't broken by most things. So if you want a 10x10 portal in a round quartz block frame, or any other shape or material, this would allow you to do that.
No. I don't really know much about how the pocket edition handles the nether.
This is just a normal flat swirly purple portal block, just like the portal blocks already in the PC version. The only difference is these don't vanish when there's a block update, and they're immune to normal damage like explosions.
Simply something that looks visually like this, but teleports you to the Nether when you touch it.
No, this would just look like the portal already in the game. Unless you put it in a different frame, or a player tries to destroy it, they would never even know it's a different block than the normal portal.
This could easily be added in, but I don’t see why. It just seems pretty useless, sans the two examples you gave which still don’t seem like that common of a problem.
Many people have talked about how they'd like to create portals in whatever shape and size they want. This would allow it.
Want a 100 meter tall portal embedded in a wall of lava? This would allow it.
And one pet peeve some people have is that the portal is always 4 blocks wide, meaning that you can't put it in a room that's an odd number of blocks wide and have it be symmetrical. This would solve that problem too.