Can I decrypt and read a partition without booting it from another mac?
You can unlock/decrypt and read any FileVault2 volume – either system or data partition – as long as the invisible Recovery HD partition (for a bootable system partition) or a Boot OS X partition (data partition) is still present and a password is known. Both partitions are usually following the respective encrypted partition. Boot OS X is no bootable partition as the name may suggest but a 134.2 MB partition probably holding some informations about the encrypted data partition.