What is the significance of the skull of Araunah the Yevusi?

Question

Araunah the Yevusi sells the land upon which the beis hamikdash will eventually be built to Dovid HaMelech (II Shemuel 24:18). Subsequently his skull is found underneath the mizbeach at least twice (and possibly a third time) by King Hizkiyahu, as per Yerushalmi Sanhedrin 1:2 and Tosfos to Bavli 12a, and the navi Haggai.

What, if anything, is the conceptual significance of this person's skull being found, repeatedly, under the mizbeach?

Furthermore, I have several questions assuming, as Tosfos seems to, that we take this literally

1. Why was there any remains found there at all? Weren't these areas excavated, by Dovid while building the temple?
2. If it was a Araunah family burial plot, as some explain, why was only the skull described as being found? And why of Araunah who presumably could not have buried himself there after selling the land
3. Why wasn't the skull removed the first time it was found?
4. Why did this cause anyone to be tamei (see gemarah and tosfos cited above)? The Rambam in hilchos tumas meis 9:4, based on bava metzeiah 114b rules that a non-Jew only conveys tumah by direct touch. Furthermore it is unclear if only a skull of a non-Jew can convey tumah.
5. Why were they suddenly excavating under the mizbeah at all?

Answer 1

I came across this list of sources that discuss it:

שו"ת דעת כהן, סימן ריד:

יש לומר, שהיה קבור במערה עמוקה תחת הקרקע, שהיתה לו ולמשפחתו קרקע חוץ ממקום המקדש, וחפרו מערה והאריכו אותה עד שהגיעה תחת המזבח

Responsa Daath Kohen, 214:

One can suggest that it was buried in a cave deep beneath the ground which belonged to him and his family, and they had excavated the cave and lengthened it to such an extent until it had reached beneath the altar.

שו"ת חתם סופר, קובץ תשובות סימן יד:

נראה לומר, דארונה נקבר בביתו במערה סמוך לבית המקדש, וגולגלתו, רוצה לומר גלגול הטומאה, גלגלתו במובן גִלגול

Responsa Hatham Sofer, Qoves Teshuvoth 14:

It appears that Araunah was buried housed within a cave near the Temple, and what is meant by גולגלתו is that the tumah rolled [to the altar], גלגלתו is being used in the sense of גִלגול (roll).

שו"ת תורה לשמה סימן תצה:

כי כבר ידעתם שכל הר הבית היה תחתיו חלול, ובימי אחז שגברו העונות, אז אותם עכבירי רשיעי הביאו גלגולת ארונה מקברו שהיה סמוך לכותל הר הבית והניחו אותה בחלל שתחת המזבח, כי כן דרך העכברים לטלטל הדברים ממקום למקום בפרט כי המקום חלול והואיל וגברו העונות יכלו לעשות כן

Responsa Torah Lishmah, 495:

For you already know that beneath the entirety of Har ha-Bayith is hollow, and in the era of Ahaz sin was strengthened, accordingly damning rodents brought Araunah's skull from his grave which was near the wall of Har ha-Bayith and placed it within a cavity beneath the altar, for such is the way of rodents to carry objects about from place to place and in particular to hollows and because sin was strengthened they were able to do so.

שו"ת ציץ אליעזר חלק יב סימן מז:

שמצאו גולגולת של ארונה היבוסי תחת המזבח, ואם כן באותו זמן נתקלקלה הרצפה או בנין המזבח והוצרכו לחתור ולחפור שם כדי לבנות המזבח מחדש ועי"ז מצאו הגולגולת

Responsa Sis Eliezer, 12:47:

For they had found Araunah the Jebusite's skull beneath the altar, and if this is so it must have been that at such time that the ground [beneath] or the structure of the altar had degraded and they therefore needed to exhume and dig there in order to construct the altar anew and on such basis the skull was discovered.