1) The original phrase is based around C3, but the assignment to C6 does trigger the arp when played, but the entire keyboard has been octave-shifted +3 (even though Octave Shift shows 0).
2) The Arp speed is back to the default value of 5, even though I set it at 108 when saving the Arp.
3) There is a delay in the sound output when pressing all other keys (in addition to the octave shift).
Please advise if there is a better way to achieve this, or if I am missing something in the Arp setup.

You should be able to fix each of these.

We need to know the "Convert Type" you used to Convert it to an ARP - this will determine what happens when you attempt to assign it to a sound and have it play. If this is a 9-note run that you want the arpeggiator to trigger on demand, then you probably should have used the "FIXED" Convert Type. "Fixed" play exactly what you entered when you trigger the NOTE you select as the ARP control note.
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Set the PART NOTE LIMIT RANGE to C-2 through G8. PART Note Limit determines what pitches are available to be heard by the selected PART.
Set the ARP NOTE LIMIT RANGE to C6 through C6. ARP Note Limits determine what Note(s) will trigger the Arp Phrase - you want to limit it to just C6. Touching this key will start the 9-note phrase - the Fixed Note selection will mean that the Phrase will play exactly as you recorded it - it does not transpose or change scales or follow any chord intelligence - it just plays back as performed at the currently selected TEMPO.

The Tempo of the Arp is stored in the current Performance. The Arp phrase itself can be played back at any tempo, 5 through 300BPM. The Phrase has no stored tempo itself... The Tempo is always derived from the MODX's main internal CLOCK and/or external SYNC settings. On the top line of the Performance screen see the TEMPO next to the quarter note icon... tap that and either use the DATA WHEEL or simply TAP the [ENTER] button to enter your desired TEMPO. it will be stored with the PERFORMANCE.

Thanks for the response Bad Mister. I changed and re-saved the Arp type to FIXED. I then selected the Part that I want to apply the Arp to, loaded the new User Arp file, and set the Part Note Limit Range to C2-G8. I was able to correct the Tempo as well. This resulted in the phrase being played when I press and hold any key on the keyboard. When I release whatever key I press, the phrase stops. Also, I could not find where to set the Arp Note Limit Range anywhere, nor did I find that term in any of the manuals - please advise on how to set the Arp Trigger to C6 only, and also how to correct the phrase starting and continuing to loop (it does loop when I press the PLAY button when editing/saving the User Arp). If I get that far, I would also like to start/stop the phrase at various points in the song while playing other parts - if pressing C6 starts it, please let me know how I can stop it, then re-trigger it again by pressing C6.

ARP is shorthand for Arpeggio - so possibly search for "Arpeggio Note Limit". Searching for this in the reference manual ...

https://usa.yamaha.com/files/download/other_assets/2/1192582/modx_en_rm_a0.pdf

... finds pages 43 (picture of this screen on same page) and 84 (picture of this screen is on page 82). These are two screen which provide two different options for adjusting the Arpeggio (ARP) Note Limit. The first page, 43, is on the "All" tab - which shows you all 8 ARPs. This view allows for easier editing of all PARTs' ARPs at once. The second page (82 ... or 84 which describes the parameter) is a screen where you can edit the Arpeggio Note Range setting for every PART 1-8 all at once in one screen instead of going from PART to PART. This may be helpful under certain circumstances. Generally, I use the page 43 screen - dealing with one PART at a time.

What happens after you release the key is under your control. Wish the keyboard would hold the key down after you let go? There's a setting for that.

"Arpeggio" -> "Common" menu. Set the "Hold" to "On". Your choices are Sync, Off, and On. You probably have this set to off. Hold mode as ON is used many times for drum backgrounds. Where you press any key and the drums start and play forever. They play forever because another setting "Loop" is turned on. For you, you may want your custom ARP to lay forever - or maybe just cycle through once. If you want the arpeggio to play through only once, set "Loop" to OFF.

Hold Mode=ON and Loop=Off is my formula for triggering fixed arpeggios that I cannot easily play along with playing other notes. Like a note-filled 64th note harp run that happens on top of my string lines. I fire off this formula of a fixed-note arpeggio with a short key press and move on to the rest of my line as the arpeggiator continues to play the harp through.

If you want to start and stop an ARP - there's a setting for that too. It's called a toggle ARP (trigger mode=toggle). Where you would press a key to toggle (switch) the ARP on - then press the same key to toggle the ARP off. Page 13 of the reference manual covers different ways to turn on/off the ARP including trigger mode=gate. The same page details what buttons to press to pull up these settings.

Source: pg 13 of ref manual:

NOTE “Trigger Mode” is set as follows.
[EDIT] -> Part Selection -> Element/Operator [Common] -> [Arpeggio] -> [Advanced] (page 87)