The central answer to this question is that the word “Israel” is always a “masculine, singular” noun in biblical Hebrew. One of the basic studies for this is by J.J. Schmitt:
The Hebrew Bible does not know a feminine Israel. The dictionaries consistently and coherently give the gender of Israel as masculine. In the tradition the word ‘Israel’ names a people. Names of peoples are masculine, while names of countries are feminine.
This is often a helpful “diagnostic” when reading prophetic texts and explains why city names have feminine referents (see Ezek. 16; 23 for prominent examples), while masculine referents are used for “peoples,” i.e., “nations”.
The one exception would be phrases like betulat ysrael, “virgin Israel”, or the like. It is found only 4 times in the Hebrew Bible: Amos 5:2; Jer. 18:13; 31:4, 21. Schmitt devotes some technical discussion to this (p. 119), considering it either to be an “appositional genetive” (cf. Gesenius-Kautzsch-Cowley) in which the gender of noun and adjective are not shared, or as extending the “city” imagery present in each context. However you slice it, it remains somewhat “novel”, and in no way typical.
Furthermore, it does not matter that “Jacob” lies behind the name “Israel.” And where English translations offer “them,” it is because they are more interested in accommodating to English style (making concessions to the “target” language/audience) than representing the pattern present in the Hebrew text (the “source” language).
 J.J. Schmitt, “The Gender of Ancient Israel”, Journal for the Study of the Old Testament 26 (1983): 115-125 (quote on p. 116). Schmitt has written a number of related articles subsequently, the one most closely connected to this question being: “Gender Correctness and Biblical Metaphors: The Case of God’s Relation to Israel”, Biblical Theology Bulletin 26/3 (1996): 96-106.
 For those who have access to an ESV Study Bible, the general “Introduction to the Prophetic Books” has a helpful section on this matter, “Pronouns in the Prophets”.