It could be a faulty battery but battery drain plus getting really hot usually means there is a short on the logic board. It could be a cap or an IC. If you need the data you will need to send it to a reputable repairer unless you have the equipment and experience in microsoldering.
It will more than likely be a parts problem. · Faulty charge port. · Faulty battery · Faulty top flex. Try booting with it disconnected. If it still will not boot it will be pry damage. You will need to check around the connectors under a microscope and see if any components have been nudged off there pads.
Can Siri hear you? Sometimes the assembly can get damaged while transferring. Check the 4 contacts can make proper contact with the speaker. If you cannot find the problem you will have to purchase another front camera assembly. If this still doesn’t work let us know.
The new NAND will have to be programmed. You cannot just swap NANDS even from another 6s plus. If you haven’t got a programmer the other way is to purchase one through aliexpress, you would need to give them the syscfg info (serial number, Wi-Fi Address etc.) from the original NAND and they will program it in for you so as you only have to solder it on.
Copying the battery information will not work. If you want to get rid of the message you will need to remove the original battery flex with its components and spot weld on to replacement battery. It is not worth it unless you are refurbishing a lot of them.
Since this happened a long time ago it wouldn’t hurt replacing the battery and screen to see if it will boot. If it does boot I would back up the data immediately. If it doesn’t boot you will need to send it to a reputable repairer so they can get the data for you.
Are you sure the top mic works? This is the only one Siri uses. The only thing that could maybe cause this is the region that was programmed into your new NAND. Did you copy the old NANDS SYS config info over or did you manually add it?
If the symbol disappears it is most likely a hardware fault. From experience you should return the phone before you require a hefty repair bill as I expect this symptom to develop over time resulting in no Wi-Fi at all.