Notebook Hardware and Upgrade Questions

Hello.

I just recently bought a HP Probook 430 G3 with i5-6200U, 500 GB HDD and 4 GB RAM. I have several questions  regarding this product:

1. At startup, I  wanted to enter BIOS to check some of the specs. Due to a misunderstanding, in the Startup menu, I pressed the "Update System BIOS" option at the bottom. Based on an erroneous reading of HP manual, I thought it would open first a menu with the information about the BIOS. Instead, it told me something along the lines "HP cannot find the BIOS firmware bin files. Make sure you have the bin files in some (kind of) HP/Product/New folder on your HDD or a USB flash". I pressed ok and the laptop rebooted normally.

Can anyone tell me what was that all about? In my case, nothing really happened because I had no BIOS update files on HDD or USB, but shouldn't the user be asked at least for one confirmation before HP tries to update the BIOS? Or maybe that confirmation is asked AFTER HP find the necessary files? If HP goes straight to business, so to speak, when upgrading the BIOS, that's quite dangerous, because in this case it is quite easy to accidentally press the wrong command, no?

2. I found on the net conflicting information regarding the memory for my model: one manual says 430 G3 uses DDR3L memory, another says DDR4. Which is it?

3. The memory speed listed in Computer Setup is 2133 Mhz and, the Windows indicates the same speed, but programs like SandraSoftware or CPU-Z says that the installed memory is actually a DDR4-2400. Why is that? Did HP install a memory faster than the motherboard can support?

4. If I wish to add one additional memory module, which would be best?

5. Is it possible to replace the pre-installed memory with some of my own? I mean, if I decide to take out the 4 GB module in order to make room for two 8 GB modules. Would this be ok?

6. Regarding the BIOS: Mine is version N78 01.15. Computer Setup says it was installed on 7 November 2016. But why is the 01.15 BIOS version on the HP Support website listed as released on 12 December 2016?

Thanks in advance.