As-salamu alaykum
JazakAllah Khair for your kind comments. I am only a mere humble servant of God and only He knows the true condition of everyone's hearts. We ardently seek His mercy, God willing.
Please see below my response to your question.
There is not one explicit statement in the Quran which categorically states that Iblis (Satan) was an angel. Such an understanding is only 'implied' from verses where the angels were asked to prostrate (2:34, 15:28, 20:116, 38:71, 7:11, 17:61), yet Iblis did not.
The other source of interpretation is through popular Biblical understanding. However, it may come as a surprise to many Muslims and Christians that not once does the Bible categorically state that Satan / Lucifer was an angel. This is merely an interpretation, and yet there are passages in the Bible which can lead the ardent enquirer to conclude that he was an entity other than an angel. Furthermore, there arguably are certain characteristics of the angels that could be derived from the Biblical narratives which the Quran may not be deemed to support. A full exposé of the Biblical position would be outside the scope of this response. Suffice to say, one must be prepared to challenge popular thought through the lens of its underpinning scriptural source.
However, there are explicit statements which categorically state that Iblis was a Jinn (18:50); that he has a progeny (dhuriyyati - 18:50), that he was made of fire (7:12) which is described and elucidated elsewhere in the Quran as the constituent components of the Jinn's creative essence (15:27 and 55:15) [1]; that he would be destined for hell (7:18) which is a just requital for those condemned from mankind and the jinn (7:178) and many other verses which clearly alight the reader of interdependence between both mankind and the jinn (including Iblis).
Implicit statements of the Quran must be viewed in light of the 'explicit' statements of the Quran (3:7)
With regards the order of prostration to the angels in which Iblis was included by proxy is better understood in light of verses 34:40-41 where it is clear that God does not necessarily communicate directly with the Jinn but uses the angels as a proxy. From these verses it is clear that God knowing full well who the culprits were, nevertheless asked the angels instead.
034.040-41
"One Day He will gather them all together, and say to the angels, "Was it you that these men used to worship?" They will say, "Glory to Thee! our (tie) is with Thee - as Protector - not with them. Nay, but they worshipped the Jinns: most of them believed in them.""
It is quite possible that the angels are used as a proxy for communication as they are tasked with decrees (97:4) and governance (ranks 37:164-166, exalted assembly 37:8, guardians in life 13:11, in felicity 16:32, in punishment 74:31) of mankind and the Jinn. Note that hell (74:31) will be the abode of the condemned from both mankind and the Jinn (6:128, 11:119, 32:13)
At no place in the Quran are we informed of the creative essence of the angels, whether they are light or otherwise, whether they have the capacity to refuse God's command outright with arrogance (istakbara 2:34) like Iblis (even though they may ask questions 2:30), whether they have any progeny or whether they have any gender (43:19) which is explicit for humankind (4:1) and implied for the Jinn (55:56).
Finally, The Quran clearly states in another verse that angels cannot be 'arrogant' (istakbara / takabbara). Please see verse 16:49. Yet, in another verse, the Quran also clearly states that Iblis was 'arrogant' (istakbara / takabbara). Please see 2:34. Therefore, Iblis simply could not have been angel before he became arrogant or after.
I hope that helps, God willing.
REFERENCE
[1] The Quran describes the Jinn's creative essence as 'nare-sumun' (15:27) and 'mariji-min nar' (55:15). This is not literal fire. There is some semblance of fire but it is also 'sumun' which gives one a sense of ferocity, intensity and 'marij' which means confused, impaired, mixed and something which is unsteady. It is a description used by the Quran to best describe to a 7th century Arab audience a substance which they may not be familiar with. Clearly this is not the same substance by which mankind was created (15:26).