(a) If one is praying in a Minyan of exactly 10 men and he has not completely finished his Shmonai Esrai (i.e, has not removed his feet), does the Shaliach Tzibur have the right to start Hazarat HaShat”z?
(b) Is a Cohen allowed to ascend to Duchan well after Reitzei has started?
a) The Shaliach Tzibur may begin being that once he has finished his Shmoe Esrai he may answer Amen to the tefilla.
b) A kohen may begin his ascent until the end of “Retzei”, meaning until the last word of the bracha: “ltzion”. The ascent can be completed until the end of Birkas Hodaah: “Naeh Lhodos”.
See Megilla 18a, Shulchan Aruch O:C 128:8. See Pri Megadim Eishel Avraham ibid. 12, that in this regard the Amen following the bracha is not part of the bracha and at that point the kohen may not go up. See Mishna Brura ibis. 27 that the ascent may completed until the end of Birkas Hodaah.