"Q1. How likely is it that something affordable (Like Schiit Mani around $130 USD or Mofi Studio?) will sound equal to or better than the phono stage of the NAD 7220PE Receiver that I am presently using?"
Funny you mention NAD and Schiit. I own 7225PE and Mani. NAD is good for what it is but just for casual background music. Everything about it is entry level. You can do much better today.
The Mani is simply an amazing phono preamp for the price.
Before that I had the $25 Behringer PP400 with a modern tube amp. While very good for $25, switching to the Mani was a significant upgrade in every way, using the same tube amp.
My suggestion is relinquish the NAD to a 2nd system and upgrade your amp and the Mani.
If budget is tight look at the Cambridge Audio Topaz AM5 Stereo Integrated Amplifier and Behringer PP400 phono or Onkyo A-9110 w/ built in phono.
https://www.amazon.com/Cambridge-Audio-Stereo-Integrated-Amplifier/dp/B078H7ZP8T/ref=sr_1_7?keywords...
https://www.amazon.com/Behringer-Microphono-PP400-Ultra-Compact-Preamp/dp/B000H2BC4E/ref=sr_1_13?key...
https://www.amazon.com/Onkyo-A-9110-Integrated-Stereo-Amplifier/dp/B07J2S755K/ref=sr_1_12?keywords=i...
For a better system add the Mani to a better amp.