You do realize that the screen is totally dead, there is nothing, like it doesn't have any power or anything. So, to me, any change would mean something on the screen materializes. That's why I can't understand how piggybacking can work with two or more bad chips, unless I start replacing them as well.
What KC says is correct. You may not get a full screen, maybe only one character, maybe a beep. It is too hard to piggyback more than one chip at a time unless you tack solder them on. Remember you need all 16 pins in contact at power up when the RAM is tested. And you need to power cycle each time. If you are OK with soldering, go ahead and replace all 8 chips of the LOWER bank. However if one of the new chips is bad, you may gain nothing and still need to troubleshot further.
You are now asking the right questions which shows you are learning. 2K bytes is an unusual way to describe these chips. They are organized as 16K by 1 bit. So you need eight to get a full byte wide 16K. Only worry about the lower bank ( 0 to 16K) as they cover the zero page which prevents boot. The upper bank (16K to 32K) just provide more RAM and are not needed for boot.
Remember as KC says the piggyback is a quick & dirty test that may not help, but will save a lot of extra soldering if it does work.