Used circa 2012 Mac Pro cmd-r doesn't work. Option key only shows internal HD. I have a valid tested USB HD attached trying to get it to boot

What?

You cannot hope to manually place a MacOS image on a drive and boot from it.

To install MacOS, you use the correct MacOS Installer. There are Base systems, drive interlocks, checksums, protected directories, Volume Blessing. No mere mortals keep track of everything needed.

You place the MacOS image somewhere accessible on your Mac, then run the command contained in the image to copy that image to a USB stick, and make the USB-stick bootable, using this procedure:

How to create a bootable installer for macOS - Apple Support

NB> if you name your USB stick exactly "MyVolume", you can cut and paste the commands right out of the article onto the Terminal command-line, no (error-prone) editing required.

There is a specific procedure required to run 10.14 on that Mac, that requires you to pass through 10.13.6 to get some required firmware updates, along with additional requirements. Without doing it in this fashion, it has no hope of working in 10.14 Mojave:

Install macOS 10.14 Mojave on Mac Pro (Mid 2010) and Mac Pro (Mid 2012) - Apple Support

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You may need to re-partition the USB drive to be GUID format with HFS+ (Journaled). Most USB sticks are partitioned as MBR and Disk Utility will keep this partitioning when erasing the drive unless you explicitly change it. IIRC even the "createinstallmedia" command will just erase the volume and not change the partition layout.

If you create the USB installer using the directions Grant linked to in this post and it would not boot, then perhaps it is an issue with your USB stick. Not all USB sticks will boot on a Mac and some may be extremely slow. Try another USB stick or if you have an extra hard drive, then you can use it as the destination when creating the bootable USB installer as described in the Apple article Grant mentioned. If this won't boot using USB, then you could then install the drive into one of the bays and it should in theory boot.

You must use the "creatinstallmedia" command from within the downloaded macOS installer to create your bootable USB installer.

Look at the screen diagram shown in this article.

Is this coming up at all? If it is, you have a Mac with a firmware ,lock. It can not boot to another drive unless/until the firmware lock is unlocked.

How to set a firmware password on your Mac - Apple Support

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As you can see from the list of computers in this article, some models from about that time could be updated to include Internet Recovery, PROVIDED they received the correct firmware update. If they are older, or did not get updated, then you would need a Snow Leopard Installer DVD from the Apple online store, delivered by mail.

Computers that can be upgraded to use OS X Internet Recovery - Apple Support

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I am with you until you re-installed the drive inside the Mac Pro. Was it booting and working fine in the external enclosure/adapter?

The only versions that will not boot are ones that requires firmware upgrades (like Mojave) or the shipped-in-the box version from a different model Macintosh.

Whatever happened beyond that point is attributable to the "surgery", or to the conditions inside the computer.

Mac Pro drive must be installed can the correct drive sled for that series Mac Pro. It must be installed so that all four drive screws are "just snag" and not over or under tightened. Otherwise, the back of the drive is at the wrong height to mate with the drive connector, and the drive can not be detected.

if you are certain it is correctly seated in the connector, hold the Option key to invoke the all-in-ROM Startup Manager. This will, over the course of several minutes, discover (by brute force) ever potentially-bootable drive, and add an icon to the display for each drive. You then select the drive you like and tell it to proceed.

The Mac notes the drive by name and additional data. The drive noted as the startup drive can be removed from one bay, moved to a different bay, and still boot the Mac on next Startup.

But if you remove the drive from bay 2, re-write its MacOS, and return it to bay 2, it will not boot automatically because it is NOT the same drive, and must be re-selected as the Startup drive..

Sierra should work well: https://support.apple.com/en-us/HT208202

Blinking file folder with a question mark can be caused by two completely different things:

1) the drive last-saved by "Set Startup" as the Boot drive is not present or completely not bootable.

Partially bootable will get you the "Prohibitory" (No Left Turn) sign.

2) the NAME of drive last-saved by "set startup" as the boot drive has been clobbered or forgotten.

This can be caused if the power has been dropped since it the Startup Drive was last set, IF the 3 Volt non-recharging long life Lithium Coin cell Parameter Backup battery has died (declined below 3 Volts).

The boot drive does mostly random reads from dozens of different files, which do not benefit from RAID. Best performance for the entire System will come from a low-latency Boot drive drive such as an SSD drive.

Apple discourages the use for RAID for the Boot Drive. There is no support for making the boot drive a RAID set.

Best RAID performances comes from the second and subsequent reads from the SAME large file, without anything intervening. For best performance, Source and Destination files should go to different drives.

Remember that RAID is not Backup, and a RAID set still needs to have a backup.