May one also eat directly under the area being hovered by non-kosher Sechach in a case that the Sukkah remains valid?
If the non-Kosher Sechach contains a dimension that is less than 4×4 Tefachim [i.e. 32 cm], it is permitted to eat under it. If it contains a dimension of 4×4 Tefachim, it is forbidden to eat under it, and he must eat under another area of the Sukkah.
 Bach 626 in name of Ran 5a and Rav Hamagis 5/12 in name of Achronim; P”M 626 A”A 4; Kaf Hachaim 626/20; M”B 626/11
Opinion of Admur: Admur throughout 626 makes no mention of this ruling and seems to imply that one may always eat under Kosher Sechach that has invalid hoverings over it so long as its detraction leaves enough shade in the Sukkah. See also Admur 626/15 that makes mention of eating udner the non-Kosher area of Sechach only in the case that the non-Kosher Sechach is under the Kosher Sechach, Veztaruch Iyun!