Connectors are replaceable by microsolderers, however it is often signs of troubleshooting something else. On a 6 plus, connector damage commonly occurs during troubleshooting when the underlying problem is the epidemic touch disease. Definitely repairable for data recovery, but with multiple problems it might be more economically viable to just replace this older device if data isn’t important.

IPad mini digitizer connector gets damaged from unplugging the screen by grabbing the long side of the connector. Grab it by the short side and you’ll never have this problem. once the connector is damaged, it is a straightforward job for mail in logic board repair or your local microsolderer. It is not a beginner job. once the pads are gone then it is a complex job that is quite challenging, but still doable for experienced microsolderers—although perhaps only for data recovery

Not in any functional way, no.

Free! Just go to iforgot.apple.com and have them answer the security questions to reset the password. If they can't do that, then it is a parts iPad.

They will look for broken glass and frame damage and water damage stickers. They have no special diagnosis software. So if it looks beautiful then it's in warranty. If it looks scuffed up or any cracked glass then it is not in warranty.

Touch function is independent of the red screen/proxy sensor problem. This is either damage to the screen itself, poor connection, or damage to the touch connector on the board. All are common

Pretty sure this is normal for some MacBook models. I've heard Louis rossmann reference this behavior a few times on his YouTube channel

I'd be stunned if this is bad battery, more likely it is either a short on the main power rail of the logic board, mechanical damage to the charge port, a bad charging permission ic chip or a bad NAND flash memory chip. None of these are DIY fixes so a trip to your closest microsoldering board repair shop is likely your best bet.

That depends. Heat = a short. It is common for iPhone 6s to develop an instant short on vccmain as soon as it hits water. Attempting to put charging current into the board or even battery current is a great way to burn up all of vccmain on the board. Most microsolderers will not address a wet board that has been exposed to voltage after being wet outside of data recovery so this is DIY repair. There are great forums out there that can help you learn board repair but it depends on your interest and willingness to invest in equipment for the job. Alternative you might find an beginning microsolderers that is still willing to look at "for the sake of the phone" water damage repairs. Find out the cause of the short and remove that component. That's your next step.

The thing that jumps out at me with this post is "screws tight". That's a bad idea--I have seen a case of long screw damage in iPhone 7 from overtightening screws. Don't tighten screws past what it takes to seat them just at the bottom of the bracket. No power iPhone 7 troubleshooting. A USB ammeter will tell you if the phone is pulling charging current as normal or low. A D.C. Power supply or known good charged battery will tell you if the phone is able to boot and not charge or not boot. You can also eliminate variables if you Take the screen off and see if it is recognized by iTunes. If it won't boot then visual exam of the board is the next step.