I mean can you restore a light wallet from a full wallet backup?
No, you can't. The full wallet backup is a very large file.
In case of a positive answer to Q. 1) then:
4) Would I better follow another route like: installing light wallet into a temporary PC => send everything from main PC to temporary one => unistalling full wallet an installing light wallet on main PC => sanding back everything from temporary pc to main PC?
This is the way to switch from full to light wallet. I would change it a bit though:
1. install light wallet on temporary PC
2. create backup of light wallet on temporary PC
3. pair wallets to be able to send blackbytes
4. send blackbytes (this requires bytes as fee) to temporary pc
5. send bytes to temporary pc
At this point I'm not sure if your backup from before receiving them will actually restore blackbytes, hence:
6. create a new backup of light wallet
7. restore light wallet backup on main PC
8. move your Bitcoins to a new address
9. link your new Bitcoin address to your new Byteball address.
Note: I haven't actually tested this.