Need an expert's advice: Wireless video on location

I'm not sure where to ask this question. My apologies if this is the wrong subreddit. Further, I am not a video pro by any stretch of the imagination. My background is in audio engineering and broadcast. I know enough about cameras to know to leave it to the pros.

We will be broadcasting live video from a music festival for the second time. Last year, we used a website (don't have the name) to broadcast. It allowed us to plug our cameras to computers and stream video live. One account acted as the 'master' and we could switch to other cams on the same account. They called it something like corespondents, or some sort of TV news term.

Our setup:

We have 2 fancy Sony HD cameras that, thankfully, had composite video outputs. We mix concert audio, crowd mics, and two talent microphones to assemble the broadcast. CamA remains with the presenters and near our audio and computer gear. Mixing the audio and video from this camera went very well. Looked and sounded great!

The problem:

We cut to CamB when a band is playing. The website we were using did not allow us to mix audio and video from separate sources. For instance, video from CamB and audio from the computer CamA is hooked up to (which was the nice audio from the mix).

I believe our only option is to ditch the video mixing capabilities of this website and do it ourselves. Basically, send video from CamA in to another computer, maybe even the same computer we use for streaming if there is some sort of dual channel video capture card that we can use with software to do a video mix. We send CamB up to the stages, and they have either a device or a small laptop we can plug the output in to and stream it over to where CamA is, pick it up on another computer, and then feed it in to the computer we're doing the video mix on, at which point, we'll be able to use the nice audio all the time, and mix video properly before streaming.

My question:

First and foremost, is this the correct way to do this? I feel like I'm reinventing the wheel here. There could be all sorts of neat gadgets, software, or other solutions out there that I just don't know about.

If we want to video mix on a PC, maybe even do some rudimentary color adjustment, titles, etc, is there a single piece of software designed for this?

If we wanted to stream video for only three days to less than 1000 people, whats the best service/host to use?

Budget is a huge issue. We can get a few PCs, no problem. Software based solutions are probably preferred, I think.

Can anyone help? How would you do it? If you could tear our rig apart and build it to your liking, what would you use? Thanks so much!

TL;DR Can we stream video a few hundred feet wirelessly for cheap? Better to use a laptop in a backpack or some neat device for this? Any cool broadcast video mix software out there?