I suggest approaching the issues as follows, as a beginning point:
1) There was no clear monotheist/polytheist dichotomy during the Patriarchal period, those are modern categories. A better way to start would be to ask what did the ancient tribe(s) consider appropriate methods to communicate with and worship their specific ancestral G-d, who led their forefather Avraham out of Mesopotamia? The book of Bereshit is the primary source of evidence (considerably more detailed than the few short inscriptions discovered by archaeologists). It describes the Patriarchs erecting sacred pillars and trees (e.g. Ber. 21:33, 28:18, 35:14), like their neighbors. In addition, when Yaacov discovered idols (clearly related to the worship of different gods), he buried them (35:1-5), suggesting that other gods and their cultic objects were distinct and not acceptable, yet he did not destroy them as Moshe later required (Dvar. 7:5, 25; 12:3). All this indicates that the tribe did not accept other gods besides G-d, yet at the same time there was not a complete break with the religious practices of the surrounding peoples.
2) Bear in mind that a number of different tribal groups throughout Syria/Canaan worshipped various deities with names such as El, El Shaddai, El Eloah, El Olam, etc. The name "El" has been found in Northwest Semitic inscriptions of Phoenecians, Hurrians, and Ugarites, referring to a supreme god. The Patriarchs worshipped at a variety of these local shrines and sought oracles from them, alongside the inhabitants (e.g. Ber. 12:6, 31:13, 35:7, 21:33, 14:13). Avraham even gave tribute to Melchitzedek, a priest of El Elyon (14:18-20). There is no indication that they, or the text of Bereshit, considered that these were shrines to foreign gods, but rather were all part of a similar religio-cultural zone that also directed worship to G-d.
3) The Yisraelim in Mitzrayim are described as having distinct religious beliefs to the Egyptian majority (Shem. 4:30-31, 5:2-3).
4) Modern scholarly views tend to consider parts of the book of Shoftim the oldest extant text of the Tana'k, rather than the Chumash. Considering this book on its own (assuming no Exodus etc), the society described does appear polytheistic (2:11-23) yet side by side with that are descriptions of wars and political oppression attributed to that very polytheism, suggesting ambivalence towards the practice, at best.
5) The words for 'god' are often used loosely: they were also applied to particularly powerful/blessed/important people (Shem. 7:1; 21:6, 22:7-8; 1 Sam. 28:13; Teh. 45:6, 82:1). (This appears in much later texts too, where other people, such as Henoch [Heikhalot, 3 Henoch] and Rashbi [Zohar 1:223a; 3:59b, 201b] are described as 'HASHEM'/'G-d'.) These designations do not signify that the Yisraelim actually worshipped these people/entities as deities.