Well as per my understanding
You can see OCR File by using Asmcmd as this is a normal File stored in Diskgroup inheriting its redundancy as well. It keeps All resources information and there state.
The "voting file" is not visible per se in asmcmd ?
Purpose of Voting disk to maintain node membership information and ensuring node availability using (Disk heartbeat).
Secondly voting Disk are required by cluster During startup and it get accessed before Asm instance is up using KFED utiility.These voting disks are special type of file which neither get stripped nor inherit redundancy of DiskGroup(For eg in case of normal redundancy diskgroup 3 voting disk are created each one on separate Disk ).That is why you cannot see them using asmcmd
voting file is merged inside these OCR backup files backupXX.ocr?
Yes Earlier in 10G we has to take backup of voting disk separately using dd command which also had risk of corrupting voting disk so to deal with such problem Oracle 11G Automate Voting disk backup using OCR.
Why 2 separates files (one for "node membership" and another for "cluster and RAC DB config information") ??
Voting disk and OCR does not contain same information ocr contain resources information of cluster with dependency. while voting disk maintain Availability of node in cluster also escalate kill request to a node.in case of missing heartbeat.
Hope it helps