Martin

When I open aplications menu the screen goes unresponsive and this weird error pop up SysHeap_Debug_Free: Heap Cracked...

Its easier to buy new motherboard than attempting to fix it

Yes you can if you can find all parts. Only thing that needs soldering is battery on to logic board. But you can find these iphones cheaper on ebay

Silly question… Yes it fits but you cannot use it because iphone 5s screen has difrent type screen pins(Because the Touch ID) but if oyu put 5c screen in it it maybe even work. Not worth it.

Maybe you bent the digitizer pin when you replaced the screen but when this hapened you have to buy new logic board. This was best I find iPhone 8 Plus Bare Logic Board

The Best I Could Find https://www.ebay.com/itm/Nintendo-used-new-2DS-XL-Motherboard-Main-board-Replacement-Repair-Part-OEM-/222809810390

(1) If You Can Go To Home Then Read My Advice. (2) Go To Home (3) Open Up Settings (4) Go To Phone Information (5) Click On Build Number Repeat Repeatedly (6) Open Developer Options (7) Find Option Demo Mode (8) Simply Turn It Off Update (02/27/2019): Then you can try “The Microsoft Lumia Way”. Hard reset your phune by recovery mode

No. Its bearly impossible. It requires so much microsoldering and custom rom wich makes cheaper and better option to buy ipod touch 6 generation that costs much less than all this effort you put to replace ram chip. Also its bearly impossible to find better ram chip and ipod touch 6 has 1 gb of ram. you can also move all your content from old device to new one. So i suggest buying ipod touch 6.

I suggest you to replace sd card slot by replacing . Here is new sd card slot https://www.ebay.com/itm/SD-Card-Slot-Socket-Adapter-Connector-L-R-Flex-Cable-For-Nintendo-DSi-XL-Console/112555269944?hash=item1a34d11f38...

Reset BIOS if possible

Hold power and volume keys down if you are booting. This should boot to recovery mode. Scroll with volume keys and press power button if you reached to factory reset option. Update (01/12/2019): Sorry. Hold volume up and power button only.