Why is "ha'Geirim" written full with two 'Yud'im?
'Rashi': When David counted them (refer to 2:16:2:1 and the note there), the first Yud is omitted; now that it is the time of action (to build), it is full. Similarly, "v'Ha'alisem Es Atzmosai 1 " is Chaser (without a Vov) in Bereishis 50:25, and it is full when they did so (Shemos 13:29).
Texts of 'Rashi' that say that "Itchem" are mistaken. In Bereishis it says "Eschem", and in Shemos it is Chaser! (PF)
When was "Acharei ha'Sfar"?
'Rashi': It is above (Divrei ha'Yamim I, 22:2) "va'Yomer David Lichnos Es ha'Gerim 1 ."
The verse does not explicitly say that he counted them. It does say that he appointed quarriers from them. (PF)
Why did Shlomo count them again, after David counted them?
Radak: Additional converts were added [to Yisrael] after Shlomo became king. Perhaps these converts are the remnants of the Emori, Chiti, Perizi and Yevusi, like it says below (8:8) that Shlomo imposed taxes on them to work, and they built cities. They were called converts because they did not serve idolatry. [Yisrael] did not let them serve idolatry in the days of David and Shlomo, when Yisrael overpowered. On this condition they let them stay in the land! They made them quarriers, carriers and workers.
Malbim: These were in addition to those that David counted.
