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I have a bootable USB (ADATA Superior Series S102 Pro 16GB USB 3.0 Flash Drive (AS102P-16G-RGY)) with MultiBootUSB (multibootusb.org) with non-persistent Ubuntu, KALI Linux, ParrotSec OS, Arch Linux, and Trinity Rescue Kit. However, if I boot any of those OSs', and then remove the USB drive, any programs that I haven't run so far will fail to run, the display will start flickering, and it will crash and show lots of cmdline outputs like:

[ 10.737654] cannot access <whatever>

Is there a way to load the entire OS (and all programs, files, etc.) from the USB to RAM so that it can be unplugged after the OS boots, without losing OS functionality?
I've already tried the toram thing, the "RAM mode" option, and the "Load system to RAM" option.

I'd prefer a solution that works for all aforementioned OSs.

This post may be related to your issue. If you want to create a custom Linux instance that loads boots into RAM this post appears to be a good start. You may also want to reference this post as well for additional information on the topic.

To answer your question, yes it is possible to create a Linux Live Boot that can be loaded into RAM. The operating systems and setup you are using does this. If you are looking for a guide on how to set up your own Live Boot, you can reference the links in the beginning of this post and conduct some research to figure out what works best for your use case.

However the second part of your question references removing the USB drive after your Operating System is loaded. You can set up your operating system to load into RAM allowing for removal of your USB device but any applications located on the device will not load as you found out. This is because they have not been loaded into RAM and the necessary files to start the application are still located on the drive that you removed. There is a way to do this but...

Is there a purpose to removing the USB device while you are using it? Setting up everything to be loaded and run off of RAM can work but without a drive to write changes to nothing will be persistent. Once you shutdown a computer that you have used a Live Boot on and removed the device there will be very little if any trace of the Live boot operating system. Once powered on and an operating system or some other kind of input is loaded into RAM all trace of the original Live Boot is gone as far as I know. There is no need to reinvent the wheel here, your setup does what you are asking just not the part about removing the drive while still in use.

I have found a solution (may not work for all distros):
Where it says "Try Ubuntu before installing" or "Try from this live CD", just press "E" to edit the kernel parameters. Then, there should be a line that ends like this:
quiet splash ---
or maybe
quiet splash hostname=ubuntu ---
Add toram (or toram=yes if that doesn't work) to that line, before the dashes, so it reads:
quiet splash toram ---
(with or without hostname)
Press F10 or Ctrl + X to boot.

If it worked, then either the desktop or the file manager should have the USB mounted as a drive. Right-click and click "Eject", then remove the drive.

i managed to make my asus laptop operate just fine as long as i either removed the stick after it was put to sleep and even hybernate mode.. even once with battery dead.. recharged opened lid like nothing happened O_o... complete luck or mistake on the last.
kali 2.0 sana live persistence and luks persistence
asus model s550c series x64. only been able to reproduce result 5 times and only with laptop in full sleep and hybernation due to low batt. values