Yes. All the data will be lost because the memory hard disk is on the motherboard.
No. If we do that then the iPhone will be stuck into DFU mode and forcing you to restore (Fully erase) in iTunes. Because the data is encrypted and bound into that CPU. So the answer is NO if we just move the HDD into another device.
The motherboard can be repaired if you send it to a real specialist like us. But let say if the motherboard broken into a few pieces, we still can recover the data by transplanting the CPU and memory HDD to another motherboard. As long the CPU and memory chip is not broken then the data will be safe. I wrote an article about a transplant job before. The link at below.
https://www.iphoneservice.com.my/2019/03/damaged-iphone-data-recovery-iphone-6.html 
6
Yes. Because the Apple server recognizes your iPhone by Serial Number and IMEI. The serial number and IMEI are on the motherboard.
Yes for the serial number. The serial number is programmed into memory chip and we have tools to rewrite it.
But NO for the IMEI. IMEI is on the baseband CPU and it’s unprogrammable.
They have replaced my motherboard (not iphone store) because iphone store the cause to repair is much expensive than the phone price (iphone 6s). The problem I had was iphone gets hot and not fixed even after I have done dfu restore. my question is do I have to worry about my old motherboard? Do I need security step or anything?
Hi, iPhone 6s get hot is easy repair if you send to a real specialist like us. If you need our help, just check our contact page for the address and phone number. Our cost repair won’t be expensive because we adjust the price according to the 2nd hand iPhone value.