Hi litfoo. Welcome to VoxCommando. I will attempt to answer your questions in order:
Would be looking to use the VoxWav app on my Nexus 5 - is this model a good smartphone for good mic performance?
Probably, but the best way to find out is to try it. You can try both VoxCommando and VoxWav for FREE, and it sounds like you already own the phone.
If I used the VoxWav app on my Nexus 5 android phone, how would I perform the speech recognition training? Do I do this on my smartphone or do I have to do it via my PC running Windows 7?
Training must be done with a microphone connected to the PC. You can't do training using VoxWav. Depending on your accent you might not need to do any training though.
I understand VoxWav mic app is activated via pressing a button or tilting the device - Can VoxWav be used in conjunction with 'OpenMic' or 'Autovoice' (both Andriod apps which try to create the 'hands free' experience?
I don't think so. I am no familiar with those apps, but I assume they are using Google's cloud based speech recognition engine. I'm not really sure what you mean by "in conjuction" though so maybe you need to explain a bit more what you had in mind. [Autumn 2014 edit: search forum for "autovoice" or "tasker" to see how users have been integrating VC with Tasker and Autovoice more recently]
Will install the Voxcommando on a PC, that is always on, that I control remotely. XBMC is installed on a Raspberry Pi connected on the same LAN - any issues that the PC is being controlled remotely and not currently connected to speakets etc.?
Not really sure what you mean by "that I control remotely". If you want to have TTS voice feedback from VoxCommando then you either need to have some kind of audio output connected to the machine running VoxCommando or you need to find some other way to send the audio. VoxWav pro supports Text to speech output, where any Text to speech performed by VoxCommando can be sent to your Android phone and it will be spoken using the audio output on your phone, and whichever Android based TTS voice you have installed on your phone.
On jitterjames YouTube videos, I noticed that a female voice responds with weather details, confirmation of actions and asks questions etc - sorry if this is a stupid question - can someone explain where this voice is coming from and how do I set it up? Is this feature part of the Voxcommando program or something?
This is called Text-To-Speech or TTS. The voice is coming from the machine running VoxCommando. In my videos the machine running VoxCommando is connected to my Onkyo receiver. XBMC is running on the same computer. I recommend you read up a little bit on the forum, wiki etc. to familiarize yourself with this TTS concept.