electrical tape will be fine. I’ve opened up iPads before that have been repaired(screen replacements) and put them back together without those covers and they all didn’t have any issues working. The only thing that is quite important though is the battery screw.
If your computer still has a standard hard drive or fusion drive, I’ve noticed quite a lot of them running very bad on BigSur. I would suggest either replacing the hard drive of the fusion drive with a 2.5 inch SSD or going back to macOS Catalina.
Go to System Preferences > Startup disk > Select your boot disk and then press the restart button. your iMac doesn’t currently have a default hard drive selected that’s why it’s taking ages to detect it. if it’s currently taking a while to start up hold down the option key and select your drive.
Just to be sure, but you are using a wired keyboard, right? If so, hold down the option key until your boot drives show up, with your USB installer plugged in. Your iMac is currently trying to boot off the internal SSD, which is currently broken because you took out the hard drive. If the USB doesn't work in that boot menu, you will see an option to enter in the wifi password or use ethernet, and the network recovery option will show up after a couple of mins.
Acetone does a very good job and removing glue but just make sure you don’t get it on any plastic components in the iPad like the speaker, a plastic spudger is quite useful as well. if you have a black screen you can use a black marker to cover up the scratches.
You will notice a pretty big difference in performance getting an SSD in an older iMac. Using a tool like the one that’s including in this kit makes it very easy. If your iMac is cold to the touch before starting area warm the edges up a bit with a heat gun. https://australia.ifixit.com/products/im...-intel-21-5-2012-2019-adhesive-strips Make sure you back up your data to time Machine before starting as restoring your iMac with a split fusion drive can be a pain.
i’ve done many iMac SSD upgrades and it makes a huge difference, if it is a 27” iMac with the User upgradable ram slot. I would recommend just doing the SSD first and see if just doing that suits your needs, If you do decide you need more ram easy enough to upgrade that. If you have a 21” iMac where the ram slots are internal on the board. I would upgrade that to 16gb while you're already in there. But an SSD will make your computer feel better than new.