Help Identifying Some Stuff On My R32?

[GSX-R35]

I'm currently plugging away on the process of restoring my Nismo R32 to the decent state it deserves to be so last weekend my friend and I stripped out the interior in preparation for new carpet, sterero, Dynamat, and replacement interior parts once it gets back from a respray. While we tore things down we found some stuff that we weren't sure about so I was hoping someone here could shed some light on them. I'm no expert mechanic myself, just some poor sod who likes to futz around with cars so please forgive me if some of my assumptions are off base.

First was this black plug underneath both seats:

Each seat had one underneath but they weren't connected to anything. We had theories on what they were but I was hoping someone could tell me for sure so we don't just ignore them if they fulfill some vital function.

Underneath the carpet on the passenger side we also found this:

I was worried it was an anal probe installed by the Grays at the behest of Majestic 12 but some googling told me it was just a sensor for the now long-gone catalytic converter so I took my tinfoil hat off. It was just loose underneath the carpet so what's the best thing to do with it? Zip tie it and erase it from memory or is there a more elegant approach?

Then we found these:

They were wired to the ECU but were loose under the passenger side carpet. There were two of the HKS Idle Stabiliizers but one was still taped to the stock ECU while the other's tape had failed and so was loose. I googled them and my understanding is they're good little bits of kit to have to prevent idle issues on R32s with aftermarket BOVs like mine. I figured I'd leave those alone in that case since the car runs fine currently.

The other box I need advice on. From further googling I found out it's a module for a Greddy boost controller and I know the car had it before in its previous life but I have no idea why the former owners decided to remove the display, controller, and whatnot and leave this module connected. Does it still do anything the way it is now? I figure at some point I'll need to bring the car to a tuner for some proper engine work and tuning and have them sort out what vestigial bits left by the previous owner can be removed - in the meantime should I leave this be and just tuck it in a safe place until I can have an expert check it out or is it currently useless and removable?

Lastly, this was hanging under the driver-side dash unattached:

What's that black box thingy in the middle? It looks OEM and it has a small bracket that looks like it should be connected to something to hold it in place but I couldn't find the spot it attaches to. My theory is that it's the module for the door chime or something similar but I'm a noob so what do I know lol. I'd like to get it back in it's proper place or remove it so it doesn't keep hanging around loose but first I need to know what it is. Also, can anyone tell me what the brown plug is for? It was loose when I removed the lower trim panel.

Thanks in advance for your help.

[r32-25t]

The first one is the seat belt wires, the second one is the cat sensor as you already worked out, just disconnect it and get rid of it

[GSX-R35]

The first one is the seat belt wires, the second one is the cat sensor as you already worked out, just disconnect it and get rid of it

Okay, that makes sense about the two plugs - the previous owner must have disconnected them when he swapped out the seats before for aftermarket ones. I'll see if we can hook those back up now that the OEM seats are back in the car and get rid of the cat sensor. Thanks for the info!

[GSX-R35]

Anyone with any ideas about the black box in the last pic? That's the one I really want to identify at the moment.