Assuming I've got the picture, I think this is how it might work.

I think you can build your own private network and become a private ISP
by sharing your bandwidth (non-profit of course), and anyone elses that cares to join you in this venture but ultimately those users you share with will be using your internet account even if you have multiple accounts to the same link.
You can do whatever you like on the private side of your network.
To setup redundant DHCP servers with another person as keen as yourself you would agree on an addressing scheme for those servers and your private user community. If your users are off-net to those DHCP servers then you would have to provide a means of forwarding their DHCP requests to your DHCP servers via a router.
When your users access the internet, they will be utilising your ip address allocated by your ISP so you will be providing NAT in your private network in order for them to do this.
If another member of your private community also wishes to share their internet access then you would have a load balancing mechanism somewhere in your network, probably on a router, plus this is probably where you will also control the allocation of bandwidth.
I do not think you can aggregate your ISP link and your peers ISP link/s
because your entry points at the ISP end are seperate and your packet disassembly/reassembly will not work.
All traffic coming from your source will be returned via your path and so too with any other peers ISP link.
I am not sure that ISPs support aggregation either. You could ask them.
Also, you cannot route any private addresses over the ISP's network. The packets will be dropped. that's just a rule in the ISP world.
I'm talking about the 10.x.x.x networks and the 172.16.x.x networks etc.
Unless you have a registered address, which your ISP already knows how to get back to, you must rely on the ISP assigned addresses.
You posted this a while ago but I am new to the forum. You probably have all of your answers and some by now.

Good luck.