That is part of the original watch OS. It appears when you press and hold the side button and then do a firm press on Power Off before letting go. However, it’s strange that a hard reset hasn’t solved the issue. What happens when you factory reset the device? Are you able to? Is it an endless loop no matter which option on which screen you choose??
Test with one stick in just one slot. Does it post? If not, try it in another slot. If still no luck, take another one of the new RAM sticks and repeat the process. The goal is to determine if you received defective RAM, or if one of your slots is potentially damaged. You may have also ordered the incorrect RAM. What speed is the new RAM? Try with one of your old sticks and one of your new sticks as well.
It sounds like the cables from your DVD player to the back of your tv are improperly plugged in. Try reseating the connections, or change where they are plugged into. Component or RCA Cables must match the colored plugs on the cable ends to the color coded jacks on your home theatre and TV
You can use any program that has the cabability of playing DVDs. VLC is the most popular program. Windows Media Player is built in to windows. I would recommend resetting the computer if it still has old stuff on it, too.
This greatly depends on your budget. I would recommend cleaning the computer out, clearing it of all dust and debris, reapplying thermal paste, and yes: upgrade to an SSD. Upgrading to an SSD makes a night and day difference in almost every single computer that previously had a mechanical hard drive.
If you cracked the button with the heat, or accidentally nicked the cable, there is the possibility of you having broken it. Try reseating the home button connector, next try a new screen. At this point, it is already broken, so it can’t hurt to try. Apple is the only one that can fix the home buttons. Try reseating the screen cables too.
Unfortunately, your model iMac cannot be upgraded to an i9 processor. The i9 was introduced several years after your model of computer was made. I believe the highest processor that model can be upgraded to is the i7-6700k. The i7-7700K will not work, and you cannot upgrade to an i9. If you are looking for a performance increase, I’d suggest putting in an SSD if you don’t have one already.
If you have the tools, you need a p5 screwdriver to take off the back. Unplug the battery and reseat the display cable. You most likely need a new screen but that wouldn’t hurt to try, since you already tried SMC and the other obvious troubleshooting steps