Am following your footsteps, testing out their DDE API before also migrating over to C++ (possibly Java if speed is not too big an issue).
Anyway, I saw your post and it has many of the questions I am asking myself, but unfortunately no replies. I've posted some of my answers below but from the perspective of someone experienced as a general USER of IB, not an API/DDE user. Hope it helps and if you could fill in the blanks, that would be much appreciated (if you did managed to find the other answers)!
Are you the beneficial owner of all executions (meaning, unlike SAXO, are all trades settled against your personal chess account)? Re: Beneficial owner, I have 2 accounts, cash (Personal) and margin (Company). My accounts with IB are not Chess-sponsored, not sure they even do this. That somewhat increases clearing/counter-party risk if that's a big problem for you? Never been a problem for me, I actually enjoy skipping the piles of Paperwork I get from my other brokers.
Have you witnessed on market hedging (i.e. have you witnessed subsequent flows from IB or other brokers as IB covers positions (long or short) relating to trades executed via your margin account?) Not sure what you mean by this, if it's DMA (and it is), there's no need for the broker to hedge? OTC would require hedging, but you're talking about physical shares right? Can't trade share CFD's if you are an AU client anyway.
Is it a true DMA platform (Their FSG talks of routing via CHX & DMM's - not ASX) + are you using their margin acct or their retail offering? Yes. I trade via the depth everyday. No DMA, no trade. Both accounts.
Have you had leveraged positions closed out at times of volatility without your consent?
Any technical issues with their API? (I know the DDE ALE is the least stable option - I plan to migrate to Java/C++ at a later date after the platform has proved itself)
Have all your orders flowed freely or does the IB compliance department actively block orders they deem as questionable? Freely
Do they provide both ASX & CHX depth? No, only ASX. But they provide full tape data (ASX & CHX) which is strange.
Do they provide level 3 data - specifically flows by broker like OM provides. No.
How to they handle a lack of available funds at time of settlement
Are there any hidden fees or traps or anything else you thing worth pointing out? Just the costs of any Market data you might subscribe to. So no, nothing hidden. IB are very upfront and provide full disclosure about what they charge, very different feel to my other brokers. They are profitable and super-competitive because of economies of scale so I'm yet to come across any "trappy" behaviour from them.
