If you’re still wrestling with this, you could always try the ‘chan-sccp-b’ extension. It sits along side Asterisk (like DAHDI). I wrote a guide to installing and configuring it with Asterisk a couple of years ago and I still use it today (in fact, I just installed a new system with 10 SCCP phones a couple of weeks ago).
The nice thing is that all of the features of the phone (even the ones that are nerfed by the SIP load) work in the native mode. It’s an option - it might be a good choice for your environment.