Is the reward for Friday prayer attained if one enters the mosque while the Imam is ascending the minbar, and when is this reward lost?

It is not permissible to run to the prayer. The Sunnah is for the Muslim to approach it walking and with tranquility. Whoever catches one rak'ah of Jumu'ah has caught it and attained its reward, by agreement of the scholars, and the correct madhhab is that of the majority. Whoever catches the Imam before he ascends the minbar has attained the reward of early arrival, because the angels do not fold the scrolls until after the Imam ascends. Striving for Jumu'ah is recommended from the beginning of the day, and obligatory upon hearing the call, due to the Almighty’s saying: ﴿O YOU WHO HAVE BELIEVED, WHEN [THE ADHAN FOR] PRAYER IS CALLED ON FRIDAY, THEN PROCEED TO THE REMEMBRANCE OF ALLAH AND LEAVE TRADE.﴾