How can I find the exact factory image version that my Pixel is using?
Problem solved, my android 10 phone is working again!
The first crucial observation was that there are something like 18 different different versions of the factory images for Pixel 3 XL for Android 10. However, there is an interesting difference in the naming convention for the first 5 of these factory images, compared to the rest. The first 5 begin with &quot;QP1A&quot; where the rest begin with &quot;QQ1A&quot;. See snapshot below for what I mean ..

Originally, I had just tried with the most recent factory image, which is QQ3A.200805.001, dated August 2020. Now, the most recent QP.. image, QP1A.191105.003, dated Nov 2019, was emerging as a candidate to try.
The second crucial observation was, looking at the factory image for QQ3A.200805.001, in the same zip file inside it that contains the boot.img, there is also an android-info.txt file, highlighted in blue in the screenshot below. So I was like, &quot;duh .. why don't I see what it says?&quot;

Here is what the android-info.txt says:

require board=crosshatch|blueline
require partition-exists=product
require version-bootloader=b1c1-0.2-6374987
require version-baseband=g845-00107-200409-B-6382632

Now, compare the bootloader and baseband versions here with the versions shown in my fastboot screen (the screenshot posted in my question above). So for this phone, I have bootloader=b1c1-0.2-5672671 and baseband=g845-00048-190722-B-5745561
Unless I misunderstand what &quot;require&quot; means, then &quot;require version-bootloader=b1c1-0.2-6374987&quot; and &quot;require version-baseband=g845-00107-200409-B-6382632&quot; clearly explain why the boot image from QQ3A.200805.001 didn't work as desired. It requires different versions of the bootloader and baseband than I had on my phone.
Excitedly, I checked the android-info.txt from the factory image of QP1A.191105.003, and the versions matched!

require board=crosshatch|blueline
require partition-exists=product
require version-bootloader=b1c1-0.2-5672671
require version-baseband=g845-00048-190722-B-5745561

By the way, I also checked another of the QP.. factory images (QP1A.191005.007, Oct 2019), and the bootloader and baseband versions also matched. Whereas even the earliest of the QQ.. factory images, (QQ1A.191205.008, Dec 2019), didn't match.
Anyway, happy to report that after patching the boot image from QP1A.191105.003, Nov 2019, and flashing it to the phone, touch has been restored and everything appears to be working properly again.