I currently have my mobile phone / NBN / fetch mighty all with Optus.
Wil my fetch service still work if I get Kogan NBN, they are offering NBN 100 for $65 and Optus only offer me this for $100. Obv I would be cancelling my NBN service with Optus. Assuming I would need a new modem also ?
I'm sure Optus will be sorry to see you go. To answer your questions:
1) Optus owns the Fetch Box (you only lease it). If you cancel your NBN then you will have to return it to Optus (they will send out a paid satchel / courier to do this on request.)
2) To be fair the Kogan deal is only for 6 months (Costing an average of $75 over 12 months and $85 a month after that). But its month 2 month so you can probably keep hopping around - one of the benefits of the NBN
3) You can keep and use your Optus modem. It should work fine with Kogan NBN. The only caveat is you might not be able to have a landline but then the Kogan deal doesn't include phone calls so that's not an issue.
4) If you want to get a replacement Fetch box I recommend a refurbished one directly from Fetch themselves (be very careful from buying one from anyone other than a major retailer as most will have been deactivated.)
5) Just a double check, but you might be getting a $10 discount on your mobile bill because you have Optus NBN. This will stop obviously.