USB mice are a bit of a weird situation. You can't use them in menus or as direct replacements for Joy Con 2 mice (so no Drag x Drive, no Welcome Tour, so on), but you can use them in some supported games (most notably at the moment Nobunaga's Ambition Awakening).
The Switch 1 Pro Controller also worked fine in the past with a third party cable (it was some sort of phone charger, I can't remember the exact model). I haven't got one at the moment, but if I get the opportunity I will let you know how it goes. However, it is likely going to work due to my Switch 1 Joy Con charging grip charging fine off all the random cables in my house in addition to the one that comes in the box.
Unfortunately couldn't find anything better than DisplayHDR 400, but I would recommend this Alienware AW3225DM instead of the Samsung as it has HDMI 2.1 support, which is most likely needed for 1440p output at 120Hz for Switch 2. The Samsung only has 2.0, so will be a bit flaky when it comes to 1440p 120Hz output - my Alienware AW3423DWF for example has a HDMI 2.0 port, and the Switch 2 only allows me to use either a 4K 60Hz input (downscaled by the monitor to 1440p 60Hz for a better 1440p display) or 1080p at 120Hz.
The main launch titles and updates (Mario Kart World, Zelda Breath of The Wild/Tears of The Kingdom, and so on) already support HDR in game.
I'm assuming you are in the UK from the instance (same as mine). What sort of budget do you have, and what size do you want? I'll have a look and see if you can scrape in DisplayHDR 500.
I'm running a QD OLED monitor, so take that into consideration with my results. HDR in Nintendo titles seems to be more focused on increasing colour accuracy rather than "pop", which will be something that is down to your personal taste. For example, the sun in Breath of The Wild turns from a white blob in SDR to a well defined sun with rays in HDR. It is definitely noticeable, but not in the same way as a lot of other consoles or PCs do HDR. However, this depends on the game, as Cyberpunk 2077 (I don't own it so have to go off what other people say) has a far brighter and more traditional HDR master that has high contrast.
I would not recommend getting a HDR 400 monitor if you can. The specifications for HDR 400 are significantly watered down (10 bit not required, sRGB colour gamut rather than WCG, high black level luminance allowance, so on) compared to even DisplayHDR 500 certified displays. If HDR is a priority, it would be worth getting maybe a lower refresh rate monitor but with at least DisplayHDR 500, as the HDR will be far better.
I can't comment on HDR at high refresh rates as I have no games at the moment that use both, although I assume it would work fine at high refresh rates.
My Switch 2 Pro Controller and Joy Con 1 charging grip both charge fine off a Samsung 25W charger with an IKEA USB C to C cable (on a sidenote, IKEA of all places do great chargers and cables, way better prices too), as well as a Samsung USB C to A cable connected to my PC's front ports. Perhaps the cables he was using were old or defective in some way? Although that stream did make me think the whole time that Nintendo (and also Sony) should just use regular AA batteries like the Xbox controllers.
Yeah, Nintendo have always had a bit of a weird mentality when it comes to their hardware. They always like doing things their way, and for you to only deal with them for repairs and service (as far as I know not even having a repair partner program to make their service easier to access). It's a shame, considering their hardware has been fairly modular and easy to open in general.
As far as I know, the charging is mostly just regular USB C PD, although with some odd power profiles (weird voltages that most non-PPS chargers don't support) and Switch 1's charger technically violating the spec through no 9V profile (the dock also does, but that's to allow the drop in functionality without the regular USB C click). Switch 2's charger complies and has a 9V profile though. This standardisation means I can power the Switch 2 dock with my Lenovo laptop charger no worries since it has a 20 volt profile at 3+ amps. Similar story for any power supply with a 20 volt profile, or one with PPS that can work in that range.
Things are this are why I am so glad for European crash safety regulations. Even an adult getting hit by an F150 must feel like getting hit by a brick wall.
I was miffed about the digital triggers at first, but it makes sense for Nintendo's main titles. In Mario Kart and Smash, for example, you need a quick and snappy button input for ZL and ZR for things like drifting or shielding, and an analogue trigger would take more time to press down. Still, for use outside of the Switch 2 it's not ideal. Hopefully for games that can use analogue triggers on the Switch 2 we see more support for the GameCube controller, since we have the new Bluetooth version for Switch Online.
Joy Con 1 is a design disaster internally. I really question how they managed to get past testing sometimes. The original board on the right Joy Con that came with my Switch was so poorly made they even forgot to fully solder in the R button, causing it to literally fall off during disassembly. As a parting gift, the casing plastics developed a crack around one of the screw holes, causing the screw to no longer go in properly. I had to hack together a shell from a parts Joy Con I bought, since it was a special Smash edition Joy Con that I couldn't get a new replacement of.