And back to the topic at hand. after testing more and reading more, now I can see why you all sounded confused about my questions, real sorry about that!
Firstly, I read up about bridging two or more NICs within pfSense, thinking that I may need it. Turns out that it can be done but not recommended; the more recommended method is to simply connect a switch to it, which I failed to consider all these while! So, all I did was to connect to the switch first, and then to the Orbi and other devices. This meant no more funky port forwarding needed for HAProxy (yes, this works too but I am trying to avoid this) and yet all devices being able to connect to the Internet and be protected by pfSense.
Then, I thought through more carefully and noted how only my guests will need only wireless connection, which means I just need to make sure they can connect wirelessly to the Internet but not to my devices. This means I need a properly working "Guest Network" function, which can be accomplished if I keep the Orbi in Router Mode.
Then also, I realised that my laptop, which is connected to the switch, can also connect wirelessly to the Orbi. This means it can be on both subnets (and thus workaround issues such as not being able to print/scan from wireless printer and my mobile devices not being able to find it)
All these knowledge made it a lot easier to solve my issues. Now I can have pfSense packages working properly with my wired devices, a proper Guest Network and all personal devices (non-guest) devices able to talk to each other whenever necessary.
Here's my new (and final?) network diagramNetwork Diagram
In short, yes, in a network, there can be more than one DHCP server and devices can have multiple IP addresses by having one per network interface
Now only one last thing left: If I were to run out of ports on the switch, should I daisy chain another switch (the cheaper option), or should I try to find a bigger managed switch and replace it (the much expensive option)? Current switch is a TP-Link 8 port Smart Switch TL-SG2008