1) You are not routing. The router is irrelevant and distracting. Your B network is really like this:
All the direct traffic between camera and cctv device will be switched by the bottom switch only. Good
Traffic from a PC to/from the CCTV box will pass through three switches each way. Okay but not optimal.
Traffic between PC and NAS will stay in the top switch - good
Traffic to/from the internet will pass through two switches (the middle one being internal to the box you call a router) This is unavoidable really.
The only change I'd make would be to pick your best switch and use that as the top switch, call it "the core switch" and have a link out to each of the other two switches. I doubt your router's switch supports trunking or bonds, and may not support vlans.
The only traffic that will go over the entire network is broadcast traffic like ARP. And for a small enough network like this, will be fine. If you had a thousand devices on the same network, broadcast traffic may be an issue.
Depending on the needs of your organisation, there are some small downsides:
any PC can go and open each camera's web interface and possibly see the imagery. Same for the CCTV thing, so implement passwords.
If you have guest devices like visitor cellphones or tablets or laptops etc, then they can see everything in your flat network. At that time you will need to consider a separate network (vlan and SSID) for them.
As for your question about bandwidth, here's my linux host running motion for 8 cameras of various ages, but with four 1280x1024 and four at 1920x1080. The time window is 48 hours.
Yes the hostname is donkey, no I don't remember why.
The flatline is the overnight low - where each camera is in IR mode and sending a lower bitrate. During the day, they're seeing more colour, and more movement.
On that basis, a peak of 4.32 Mbit/sec, that's 540 kbits/second/camera. Ballpark 600 kbits/sec and your 16 cameras to the DVR will use less than 10 Mbit combined at full noise.
Of course if you have 10 PC computers all watching 10 cameras through the DVR at once, then this number grows pretty quick, 10 *10Mbit (outbound) + 10 Mbit (inbound) is 110 Mbit/sec which is roughly 11% of what a gigabit port can theoretically do, probably more like 15-20%.
So you have quite a lot of capacity for cameras.