From what and how to make a shaper?

Question

linux

Available: network of 10 PCs. There is 1 provider, which is included in an old ADSL modem, then a switch and other machines.
Task: run the torrent to the fullest on all 10 PCs and enjoy pleasant viewing of YouTube and comfortable web surfing there.
I tried QoS and dd-wrt+DIR-320 before - it didn't work very well. Now I'm going to take some old machine and deploy pfSense or CentOS on it. Of the problems - there is no physical possibility to connect the Network-Gateway-Modem in series.
How I see the solution: the gateway, network and modem are connected in parallel to the switch. The modem has a static address - 192.168.0.1, gateway - 192.168.1.1, network - 192.168.1.0\24. DHCP is enabled on the gateway. QoS is enabled on the gateway and redirection of all traffic from network 1.1 to 0.1 (that is, to the modem).

1. Is such a scheme possible, will the shaper work and how will the return traffic go?
   
2. Is the option of transferring traffic to another subnet adequate, or can I leave the router on the same network?
   
3. How much does pfSense with its ipfv outperform Linux with iptables?
   
4. Is it even possible to set up a shaper with one network card in a switch, and without any shamanism like VLANs
   
5. Maybe it's easier to make two network cards, one per subnet and make the gateway drive traffic between the cards?
   

I hope for your adequate answers.

Answer 1

As I understand from the text - you have a router + modem. In principle, you can leave it on the same network as the gateway and not make it NAT from the gateway. Then on the router-modem we turn off any functionality such as DHCP and delegate this joy to the gateway. Thus, the gateway must be gateway (as I famously avoided the pun) on all machines. For the gateway, the gateway will be a modem router.
Total:
1) Yes, according to your scheme, the gateway will be a NAT.
2) In principle, yes, I suggested another, it's a matter of taste, IMHO. If users do not climb into the settings (such as setting the router-modem as a gateway with their hands to bypass QoS), then mine will roll, yours is safer in this regard.
3) Wins in what? As far as I know, ipfw is somewhat more performant. Well, it's easier to set up.
4) I didn’t understand
5) See point 2