Mac – Is it safe to eject the timemachine backup whilst its decrypting

Time Machine prepares the drive for storing backups when you set it up. This means, disrupting filesystem-level encryption that wasn't made by Time Machine. If you want encryption on top of Time Machine encryption, it has to be below the filesystem level, or you'll surely lose it in favor of Time Machine. If you want encrypted Time Machine backups, simply enable encrypted backups when setting up Time Machine instead of encrypting in Disk Utility.

The difference with method 1 is that, with method 1, you're not actually converting anything. You're erasing the existing data and replacing it with an encrypted partition. With method 2, a conversion process begins, which doesn't erase data, but does take more time.

The thing to keep in mind: any drive which is converted from a normal drive (HFS+) to a Core Storage drive can be reverted back to a simple HFS+ volume (non-encrypted). If you use the Disk Utility method you showed above, the resulting disk will NOT be revertible, because the disk will have never been an HFS+ volume in the first place. That is what you're seeing with "Revertible: No". If you use the Finder method (or the equivalent command-line option), the drive will be revertible.

Another thing to keep in mind is that modern versions of OS X will normally use Core Storage on the boot drive by default, even on unencrypted disks. The result is that, in some cases, "Revertible" may always be "No". You'll always be able to decrypt a Core Storage volume however, so perhaps this is insignificant for most people.

If you want to talk about what's the best option for Time Machine, it doesn't really matter. The end result is, in either case, an encrypted disk, protected by a password. To do anything to the disk you would need the password. There isn't any significant difference to either method. It's marginally possible that the disk utility method could be the slightest bit more secure, for the simple reason that this reduces the number of attack vectors. If there is any benefit, it would only be slight.