I don't believe they mention training/coaching with regards to the recording anymore I don't believe. Either way though, they don't HAVE to release the recording but agreed, if they've admitted to listening to it then why not? Most likely it's because the process is black and white and only 1 team can do it and they'll only do it with the necessary documents in place so the people you're talking to are likely not to blame (doesn't make it better though).
I think you've got the right idea though, one that not enough people in the world have which is to vote with your feet. I know both Telstra and Voda offer a 3G backup (it might be 4G but I'm not sure) in their consumer grade NBN services so if there is a fault or outage, it switches to mobile broadband data to keep you running. I can only imagine you'd get an even better solution for a business service so it might be worth looking at them if being up 100% of the time is vital.
It does sound a bit dim. My impression is that recordings are made available on request (they just take a little while). For Optus they are an essential part of the legal contract you make.
I would suggest you go via the TIO at this stage. The TIO is concerned with ensuring contracts are complied with and as such could require Optus release the recording.
It does surprise me that the contracts we enter in to a so informal. Customer Service people often make promises but none of that seems to make it through to the contract and worse we aren't even sent a contract a lot of the time. By the sounds of it making your own recording would be a good idea (and I think I'll start doing that) however the last few times I've done a new deal I always request a seperate email that just spells out the critical points. Can just be a single line but hopefully it removes any room for misunderstanding (which is good for both parties)