I wanted to reply to this a month ago, but my account was deactivated accidentally

First of all, the network.json is for initial configuration of the servers, this configuration should indeed be correct and final, otherwise a reinstallation will be necessary.
Adding more external networks after the cloud has been already deployed works fine. You actually don't need to edit the network.json for that, but you need to edit the network barclamp directly (in raw mode) and the redeploy it. This will add the new external network to the database. After that, you need to edit the neutron barclamp in raw mode and add the attribute "additional_external_networks" with respective values for your new network(s), redeploy neutron barclamp. Then you need to create that network with neutron net-create command, but please note there's a network name missing in the command example in the docs. The command should look like this (for linuxbridge):
sudo neutron net-create --router:external True --provider:physical_network physnet1 \
 --provider:network_type vlan --provider:segmentation_id 600 public2
That's this part.
Resetting your admin server to a healthy state can be tricky. It saves you a lot of trouble if you use a virtual machine for admin server and take snapshots, at least one right after the cloud has been installed on admin server and before any changes are made.
I also experienced failing connection to the admin server in some cases, it could be that your network changes are not valid, did you check them with the json-validator?
Try restarting crowbar.service before redeploying the barclamp, are all services like chef up and running?

And then there's the hard way:
If you don't have any control or compute nodes registered yet, try a clean install of the cloud. But I have had some troubles with simply reinstalling the addon etc., I had to wipe everything clean by removing all packages related to crowbar, chef, couchdb, ruby etc. But make sure to keep a copy of your network.json before deleting anything! Then I deleted all remaining files and directories manually after deinstallation, then reinstall the cloud.

Since you didn't reply yet I assume you either fixed it already or gave up? I'll leave this here anyway, maybe this helps someone else, too.