My post #7 was not really a response to post #6. It was just trying to give a more complete answer to post #1. Can an abbot be a mitre bearer? Yes, he could be, in certain circumstances.I don’t remember anyone argueing this point…however, this would not be the norm.
To respond to post #6. “An institured acolyte is not an altar server”. OK, an “instituted acolyte” is not the same is a “non-instituted altar server”. I understand this, and I think that would be clear from the quotes I provided.Well, if the quotes you talk about are the GIRM quotes, then you used the section to describe what the duly instituted acolyte’s duty, not the altar server.
On the other hand, how do the liturgical books describe what an instituted acolyte does? “Their proper ministry is to look after the service of the altar” (Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 24.) Again, no one disagreed with you here.
“… Make your life worthy of your service at the table of the Lord and of his Church.” (words of institution, from Institution of Acolytes, The Rites Volume Two, Liturgical Press, 1991, ISBN: 0-8146-6037-1, page 109).
“It is his duty therefore to attend to the service of the altar …” (Motu Proprio
Ministeria Queadam, n. 6).Ok, point?
From post #6 [the instituted acolyte’s] “role is to assist at the mass at the altar, but not the typical role that an altar server would.” Well, if it is meant that an instituted acolyte should not normally be the mitre bearer then this is not in accord with the liturgical book, Ceremonial of Bishops, page 24: “28. … When necessary, acolytes should instruct those who serve as ministers in liturgical rites by carrying the book, the cross, candles, or the censer or by performing other similar duties.
But in celebrations presided over by the bishop it is fitting that all such ministerial functions be carried out by formally instituted acolytes, and if a number are present, they should divide the ministries accordingly.” (My bold text.)(My purple text) Ok, again I ask, point? No one disagreed with you, so why post this?
So the correct (or “fitting”) person to be holding a bishop’s miter is an instituted acolyte, rather than a “non-instituted altar server”. Yes, this is correct!!!
From post #6: “Now there are times when the bishop would not have his crozier or miter, if he is the sole celebrant of a mass of lower significance.” I think this is wrong. Doesn’t matter if you think it’s wrong, it is acceptible in certain situations, such as conferences and meetings outside of the Cathedral. For instance, we have men’s and women’s conferences where a mass is held at the pastoral center, I served and the deacon of the “small word” at both of the masses. Bishop wore no mitre, only the zucceto.The absence of these has nothing to do with a bishop being sole celebrant. From the Ceremonial of Bishops: “60. In a liturgical service the bishop is to use only one
miter, plain or ornate depending on the character of the celebration.” There does not seem to be an option of “no miter”. You may be looking in the wrong book, go find another one.

For the crozier: “59. The bishop carries the
pastoral staff in his own territory as a sign of his pastoral office, but any bishop who, with the consent of the diocesan bishop, solemnly celebrates may use the pastoral staff.”