The kings were not an issue.
No, but the kingdom divided in two is the issue. (V42) Though they try to amalgamate, there are irreconcilable differences that will tear them apart. As for the "they" of verse 43, that refers to the two components of the division.
Indeed the LXX uses the plural form of kingdom....ie kingdoms in verse 42
So 'they' the two kigdoms
The presence of iron in the feet and toes speaks of the remnants of the Roman empire.
The picture of clay might speak to the more artistic and ascetic influence of the Byzantium part of the kingdom divided
A study of the fall of the Roman Empire and also the complicity of Islam is worth while.
Their conquest of the Levant and their current quest to recapture it.
.ISIL Islamic state of Iraq and the Levant.
Rev 13:14 And deceiveth them that dwell on the earth by the means of those miracles which he had power to do in the sight of the beast; saying to them that dwell on the earth, that they should make an image to the beast, which had the wound by a sword, and did live.??
Also, please be very careful of how you understand 'seed' verse 43.
זְרַע
translated as 'seed' in the Kjv, occurs only once in the entire Old Testament.
For a more informative rendering, try the Esv.