It is normal for the phone to boot when inserting the lightning cable. Your problems are most likely the screen and home button connectors. Remove the screen connectors and reseat them be extra careful with the home button connector and flex.
This is not possible. The data on the NAND is encrypted which is serialized with the CPU and logic eeprom. Even if you put your NAND in another iPhone 8 it would not work. There might be a really slim chance if you use a program called 3uTools and select restore while retaining user’s data. It should bring up the password screen again where you could have one more chance at entering the correct password but most of the time it fails.
It is hard to diagnose this problem over the internet. If this came across my desk here is what I would do. I know you have completed some of these steps. · Remove the logic board and install it in a known good housing including a known good screen and battery. (If it boots you know it’s a parts issue) · Take a diode measurement of the PP_VCC_MAIN line. (A partial short on the 6s plus can cause your issue) · Try an update. · Put some pressure on the CPU while booting. · Check the PP1V8 has correct voltage. · Replace the Tristar IC. · Check the logic eeprom. · Check Baseband PMU and CPU on the 6s plus if they are faulty it will produce your problem. If the update you performed was successful with no errors I would be inclined to say that the NAND and CPU are okay but you would need to check the power rails on these in case there is a partial short.
If the connectors are properly seated on the new screen and you have tried the old screen again and touch works fine it will defiantly be a faulty screen. Return it for another one.
You can purchase a charge port assembly for around $40 from a reputable supplier. Do not buy these through eBay as they are much more integrated than the older model phones as they have sensors that communicate with the CPU. The ones from eBay have a high chance of being faulty causing the 3 minute restart. (Buy from a reputable supplier)
It is not the speaker you need to replace. You will need to replace the charge port assembly as it has the audio lines that go to the speaker. This is the first part to replace when the bottom speaker is not working.