5) I'd say the third is likely more secure. The first is meant for quick swapping without being completely open, but if you tried to lay it sideways the door might flop open, or the drive might be dislodged if it were bumped. The third actually has mounting screw points to secure the drive (although the manual doesn't mention them and they aren't required if you want quick-swap capability, and they might not work for a 2.5 inch drive), and then the cover slides on, so you could lay it on its side if you wanted or shake it and the drive wouldn't come loose.
6) Almost no external drives or enclosures have built-in fans and they're really not necessary. None of the ones I linked has a fan, though the first one is a fat casing which is probably just empty space. A cooling fan would be helpful if you were going to have it in a very warm environment like a very warm region with no air conditioning, but even sitting on top of your PC it's not going to be tremendously hot and cause premature failure, and the normal airflow of a room at "room temperature" will be fine.
7) The first one might be very slightly easier simply because it's just a door, but the second is just a cover that slides off, and requires a button to release it so it's more secure. Once the cover is off or the door is open, they both work the same. You tilt the drive upward a bit until it's above the edge of the plastic and then slide it away from the SATA connector. Unless you tilt it up to a very high angle, there's little risk of damage, because the connector inside the enclosure is designed for this.
8) My expectation is that the electronics inside the enclosure are designed to require external power even if you're using a drive that could technically run on USB power. Meaning I don't think the control board is designed to draw its power from the USB port at all, so it wouldn't even turn on if you don't plug in the power brick. I don't have an enclosure like this handy to test with. I wouldn't recommend using a 2.5 inch drive anyway, because you can't get one in 8TB size.
9) Personally I'd get a Western Digital drive over a Seagate, but Seagate is usually a bit cheaper and you were expecting to get one anyway. (Seagate just has a bit worse reputation for reliability.) These two would both be fine. The WD drive would be a little bit faster as it has a higher RPM and the recording technology is faster.
https://www.amazon.com/Seagate-BarraCuda-Internal-Drive-3-5-Inch/dp/B07H289S7C
Higher performance at 7200RPM costs a good bit more, and you really don't need it for backups.
https://www.amazon.com/Western-Digital-WD_Black-Performance-Internal/dp/B0D1V1SNDX
So, you're just going to constantly pile up more and more hard drives containing old backups? For how long do you plan to keep them? I get needing to recover older data if you discover a mistake much later but this is an expensive proposition. If you really just need to retain backups that long, just get a larger drive instead of replacing them. Going from 5TB to 8TB isn't a big jump, but there aren't any inexpensive consumer internal drives in the 12TB and up sizes. A 16TB WD USB drive would be more expensive than building it yourself because you can't just swap the drive inside (but it's cheaper than the enterprise and NAS grade 16TB drives), but if you only ever need to buy it once and don't have to replace it, you save money over time. It depends on how many backups you can store in that amount of space and how far back you want to retain the backups.
It depends on what "dead" means. For the average consumer, shucking the drive isn't something they would be able to do, but if the electronics die but the internal drive is fine, they can just ship it off to WD/Seagate, then the manufacturer can shuck it and copy the data off the drive and send back a new/refurbished unit with their data on it. Even if there is a physical problem with the mechanical drive, recovering data is entirely possible. Depending on the problem, the data may all be fine, or there may be some that got corrupted or lost, but that's better than no recovery at all. There are companies that do nothing except recover data from bad drives and they're very successful.
The 3-2-1 rule for backups is always best, but for the average consumer, it's a solid win to get them to even do ONE type of backup. (And if they're syncing their files to the cloud with OneDrive being turned on automatically, that can count as their "1" offsite so having a USB drive counts as a second method.) Getting complete protection from things like ransomware and other catastrophic issues by having backups that simply can't be accessed is difficult and expensive. Everybody has to work with what they can afford and understand.
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