I have noticed that Archbishop Lefebvre and Bishop Williamson are frequently referred to as "Monsignor" Lefebvre or Williamson.  I fully realize that "Monsignor" is an honorary title and not an aspect of Holy Orders per se, but is it seen as such an honor, that the title is preferred even when the monsignor is a bishop?  I have noticed this more in French, but you see it in English as well.  Is there any reason for it, or is it just common usage?

It's my understanding that Monsignor is a title for a priest, not a bishop.

Well, seeing monsignor is translated as "My Lord" (and that is how you address a bishop) I don't see as it makes much difference.

But yes, you are right, it is an honorific title. Many years ago we had a parish priest who went by the title, Monsignor.

So is this to say that all bishops may be addressed as "Monsignor", whether they have been made members of the papal household or not?  You will sometimes see all of the SSPX bishops as well as ex-SSPX Bishop Williamson referred to as "Monsignor", yet none of them (as far as I am aware) ever received this title from the Pope.

Along these lines, I also recall that Bishop Patrick Taylor (requiescat in pace) of the Society of the Virgin Mary (Beckley, West Virginia) at one time referred to himself as "Archbishop Taylor", then at some point he began referring to himself simply as "Bishop Taylor".  Did he "demote himself" for some reason?  Obviously he had no mandate from Rome to refer to himself as either archbishop or bishop --- he simply received episcopal orders from schismatics and proceeded to function as a traditional Catholic bishop.  My son and I passed through Beckley this past Easter and were considering arranging our trip so that we could be back through Beckley for the TLM on Easter, but we drove out by Bishop Taylor's chapel and it was evidently closed, it had a "for sale" sign on it and was listed on MLS (real estate listings).  This was several months after he died.  Does anyone know what has become of the SVM since Bishop Taylor's death?

When researching more on this topic, it was interesting to find this:

https://www.ncronline.org/blogs/ncr-today/report-pope-abolishes-monsignor-honor-most-priests

There are usually two or more bishops in an archdiocese. The senior bishop is called Archbishop.
Maybe the said bishop had delusions of grandeur.

In answer to your first question, monsignor is a title, and is a French word adopted by the Anglophones. My Lord is the way we address a bishop. I suppose the SSPX tends to use the French Monsignor because of their French origins.

Actually, the French spelling is "Monseigneur", but the Italian spelling "Monsignor" is pronounced almost exactly the same.
It is entirely possible that (then-Arch)Bishop Taylor had a suffragan bishop.  I honestly don't know.  I corresponded briefly with Bishop Taylor online but I never spoke with him or met him.

Actually, the French spelling is "Monseigneur", but the Italian spelling "Monsignor" is pronounced almost exactly the same.
It is entirely possible that (then-Arch)Bishop Taylor had a suffragan bishop.  I honestly don't know.  I corresponded briefly with Bishop Taylor online but I never spoke with him or met him.

Oops! You are right about the French spelling. The Italian is mio signore, but it is not used for a bishop who is Sua Eccellenza.