sfc /scannow WILL NOT function

(1) The NET should be working well in 1909. KB4556799 is currently the latest cumulative update. It installed well for me, but maybe is doing more for you. How far back is your OS Build at System>About compared to the list of cumulative here...?...

https://support.microsoft.com/en-us/help/4556799/windows-10-update-kb4556799

May 12, 2020—KB4556799 (OS Builds 18362.836 and 18363.836)

Maybe it just has too much to do. It can be blocked with WUShowHide, but, especially if the computer is far back, you really ultimately want it in. I guess the fact it constantly fails & retries could be slowing the NET. Normally, when I take in a cumulative, I just sit & watch it till it's done. Also, at boot, I put an eye on "right-click Taskbar, Task Manager". If anything is at or near 100%, I let it subside before doing anything of my own.

(2) I think 1909 hadn't completed its install before you pulled the USB drive & booted into 1809 (volume 4). Finally it does sound like it has completed (except for updates), but I fear having interrupted it with all those boots to 1809 might have had an untoward effect, though I don't really know.

I like that the "volume" stuff has gone & you get a normal boot menu. If you undo the dual boot & try again & see volumes again, you will know which to pick (volume 5). When you get to the normal boot menu, it will have "Windows 10" on top probably, & "Win 1809" will still be there anyway. I would continue to boot & reboot into 1909 until it has completed its updates.

My suggestion is to undo it & retry for a better result.

I got into 1909, but had to go back into 1809 to send this. Tried multiple times yesterday and today to read and reply from within 1909 but it just won't work. Defender may be set wrong because it says this is a malicious site.

(I got into 1909 but this is what I ended doing - tried again without the USB as you said, no go, same thing happened still said 1809 even though I chose 1909 and that was the choices. So I rebooted with the USB in (before you said not to) and it gave me a new choice Win 1909 or Win 1809 Change defaults or choose other options. Clicked 1909; it worked. Restarted again without the USB and got the new options again, it probably had to restart a couple times to wake it up. )

System>About says 1909 YEAH! I hope.

That Volume window didn't show up this time either. So yes, the first one must have bee 1909. Good to know. The choices now are Win 1909 (on top) Win 1809 (on bottom). File manager has Local Disk (C:) Windows (D:); D has my old folders plus a bunch I don't recognize.

I forgot to write that yes I've been downloading since I got into it before, they're just really slow-lousy internet again which seems to be much worse I hope that's not 1909 . Troubleshooter says it can't identify the problem with the internet as usual, been through this a thousand times and have done the unplug, replug, stopped almost all the background apps and so on.

Just got have all the updates but one installed - left the computer on all day all night but 2020-05 Cumulative Update for Windows 10 Version 1909 for x64-based Systems (KB4556799) has yet to complete an install keeps saying to retry which I have tried several times. Is it really important?

(1) There's no need to boot to the USB except to reinstall or to get into its recovery environment.

(2) That /Set command is only changing the descriptionshown for the partitions in the boot selection screen & in MSConfg.

(3) To boot to 1909, choose that one from the boot screen. Check "START, Settings, System, About" to know for real it is 1909. If it doesn't say so there, something is wrong. Are you sure both choices say 1809? It sure sounded like 1909 installed somewhere. And it looks it too in BCDEdit & MSConfg.

(4) What about those questions I asked about "volume"?

(5) Have you gone into 1909 & done the things I suggested -- Windows Update, etc.? And -- if you are in 1909 & asked to reboot -- make sure the reboot is back to 1909. Same for 1809.

Edit:  Well, here is how to undo the dual boot, if you have determined the 1909 installation cannot be improved...

(1) Boot to 1809.

     (Check you are there at About.)

(2) Open MSConfg, Boot tab.

     It should look just as in your picture.

(3) Select the Win 1909 boot entry, & click Delete.

     Follow whatever instructions, then close MSConfg with the OK button. Reboot, if asked.

     BCDEdit should now show only the Windows Boot Loader named Win 1809 remains.

(4) Open Disk Management.

(5) Right-click the 1909 partition (between C: & D:), & select Delete Volume.

     It should return to unallocated space. Reboot, if asked.

That's about it. If you haven't been asked to reboot by the end, do one now anyway. Then I think we should try the dual boot once more.