The old kernel restores the wifi because its signature (3.4.0-cyanogenmod-g3a439b93663) is the same the folder name where the wifi module is located under the /lib/modules/.

How do we make a backup of a secondary rom with multirom?
Well, we cannot. What we can do is backing up the entire directory where the rom sits but not including the boot (kernel) and recovery partitions.
This means that in your case this "backup" wouldn't save you as you flashed a new kernel (different signature) which has caused the trouble.
On the other hand with this method you can free up some space with archiving unused but wanted to keep roms. Then later it can be "restored" back if you need. (That's how I kept my experiments with sfdroid and older SFOS versions saved onto my network drive.)
If you would like to do this, you just need to boot into another rom and as root create a tar.gz of the whole folder where your other rom sits in /android/media/0/multiro/roms/. (I don't recommend to do this with the rom you are actually using!)

You may also find the following useful.
Because I have lost many well configured systems when tried something new and had to start to setup everything from scratch oll over again, nowadays I am doing the following:

• On my Nexus I have a very light SFOS installation (call it SAFE) and I have another one that fully configured (call it PROD).
• When I would like to try something new (like a new kernel) I boot into the SAFE rom and in fingerterm as root I enter into the /android/media/0/multiro/roms/ folder. (This is where you can find all installed roms, so be careful!) Let's say the folder name is production where your PROD rom is located.
• Then I run the following:
  Code:

  cp -av production copy

  This will make a copy of your production rom into a new with a name copy.
• Then when you reboot, you can boot into the 'copy' and do your experiment.

This way you won't compromise your well configured rom. Then if you have tested the new stuff on the copy you can safely install that on your PROD rom too. (Otherwise if something goes wrong, you can just delete the 'copy' rom from twrp and start over.)