To be honest, I can't remember exactly why I chose the 124 rather than at least attempting the recording onboard, considering I have a half-dozen of those phone recorders sitting around. I think it had more to do with the better quality from hooking directly into it and bypassing the phone, and the fact that instead of 10 6-second recordings, I have 8 1-minute recordings. Maybe one day I should experiment with the onboard ones purely for the fact that they can be use in with any other spoken commands. On the other hand, I can silence non-alarm voices yet still have my custom sounds play regardless - vital for doorbell operation and certain other sounds. For example, my wife often leaves her phone in her car, and we work opposite hours... so if I need to reach her during the day, there's no way to - I've been meaning to program in a "Please answer your phone - I need to reach you!" message - another one I wouldn't want silenced.
Anyway - I bought the Elk 124 and the Elk 129 recorder module. I just scanned through the instructions and found the part I did different... They talk about using some WaveLoader software - but, I wasn't loading WAV's and I wasn't using a PC - so I figured out that you can get very precise recordings from any audio source with a 1/8" jack by pressing and holding the record button on the 124 at the same time as playing your source, then release when you want it to cut off. It immediately plays back what was recorded, so you can tell if you need to adjust volume or timing or whatever. Since then, I've been loading sounds just from my iPhone as desired.
As far as outputs go, the system has plenty - and I find little use for the voltage-only ones. Of course I could've easily put an M1RB on the onboard outputs, but most of my outputs are in the garage sub-panel on an M1XOVR/M1RB combo. Now I am thinking of adding more outputs onboard at the main panel, but I can always add an M1XOVR and get full outputs. The 124 came with the right ribbon cable to plug it directly into the onboard outputs making the install very clean.
The whole backfeeding thing doesn't seem to be an issue. There was another thread here where some of us dug up some really old elk hookup examples (related to now obsolete products) and they showed just hooking them both up in parallel and using a capacitor on each + and - on both sources... I'm not sure exactly what they do, but I didn't use them out of laziness - and the only problem I ever saw was during power up, when it would fire the startup messages and fire the doorbell (my CH1 sound) at the same time - but I only ever power cycle my system less than once a year, and no damage has been done... for such short periods, I doubt there'll ever be concern unless someone repeatedly rings my doorbell while my house is in an alarm state... but at about $5/speaker and going on 5 years of successfully running in parallel, I'll keep riding that $30 gamble.
I'm not sure where the whole house amplifier you reference comes into play - earlier you mentioned wanting to drive both sources through the Elk speakers; if a whole house amplifier is coming into the mix, at what capacity? Does it have its own speakers? Are you trying to drive elk sounds through a whole house amplifier, or drive whole house amplifier sounds through your elk speakers?