Modalism/Sabellianism/oneness/Jesus only is the heresy that Jesus is God who steps into the modes or offices of Father (OT), Son (earth), Spirit (Church Age). It says that there is only one person in the Godhead, so Jesus is the Father as to person, not just nature. JWs make the mistake of confusing modalism and trinitarianism (they think if they refute modalism, that they are refuting trinity, but they are not). For a JW, when they hear us say that Jesus is God, they think we are saying Jesus is the Father. They ask us why would Jesus pray to Himself, how can He send Himself, etc. Modalism is wrong. Trinitarians are strict monotheists and agree that God is one in nature. The question is if He is a solitary being (Unitarian, JW, Islam, Judaism) or a compound unity (like one cluster of grapes, one army, one family with many members). We must rely on revelation, not raw reason. WT publications have said that we cannot understand all about God, yet they criticize us for not being able to understand the trinity. Arianism=JW....Christ as creature vs Creator God. This is a condemned heresy early in church history along with modalism.
Jewish people are wrong about who the Messiah is, so why appeal to them? The Bible is progressive revelation. The OT shows us that there is only one true God (Deut. 6:4 Shema). It hammers away at monotheism in the face of pagan polytheism. The trinity is hinted at in the OT, as well as the Deity of the Messiah (Is. 9:6 cf. Is. 10:21 one true God and many false gods). Jesus is God with a face (should blow your mind and lead to worship of Him Rev. 4-5; Heb. 1:6, etc.). Heb. 1:1-3 Jesus is the full and final revelation of God. He claimed to be God and has the names and titles of God (Alpha and Omega, etc.). He is worshipped and honored as God (Jn. 20:28 Jews only worship one God). Numbers just shows that God is not fickle like men. It is not a proof text to preclude the incarnation, God becoming man in Christ, the invisible made visible (Jn. 14:9; Jn. 1:1-14; Phil. 2:5-11).
Trinitarians also have one God, YHWH (Jehovah is a name we use, but JWs are wrong to make a big issue of it in light of uncertainty about the best transliteration). The trinity is also indivisible since there is only one spirit nature of God. Personal distinctions within this one God are fully unified and fully diverse. There are not 3 gods.
Jesus worshipped the Father, not Himself. He was also worshipped as God by the early church. Jesus is equal with the Father, but not the Father. As the God-Man (one person with two natures), He would worship the Father (modalism might say He worshipped himself, but that is false since Jesus is not the Father despite sharing the same eternal essence).
Jews and JWs (modern Pharisees) are wrong about the identify of Jesus (Jn. 5:18; Jn. 10:30-33). The Scriptures testify about Him yet they refuse to come to Him for life (trusting WT indoctrination/error instead). Unless you believe that 'I AM', you will perish in your sins (Jn. 8:58 is Jehovahistic identity).
It is telling that the NWT twists all the Deity of Christ verses (there are about 40, but only 6 typical Arian verses that we can readily answer Rev. 3:14; Jn. 14:28; Col. 1:15, etc.). The WT booklet about the Word, who is He according to John? is correct to observe that the correct translation of Jn. 1:1 means our salvation. The NWT 'a god' fiasco is indefensible grammatically, theologically, contextually (as any Gk. scholar of note as reminded the non-scholastic, rationalistic WT). Jn. 1:1 refutes modalism, Arianism, polytheism and is consistent with the triune view alone.
I appreciate this is hard for a JW to understand (just as it was hard for for Jews and Muslims to understand, to their eternal peril), but many have come to this biblical view by just reading the Bible. The WT puts up straw man caricatures of it and knocks it down (I would also reject those views such as 3-headed god, 3 gods in 1 god, Father is the Son modalism, etc.).
I would die for these beliefs and know that it is a salvific (essential doctrine for salvation) issue. It is the crux of the matter. I may not be popular here for defending it, but it is truth, Truth that sets free. Many JWs deep down know that Jesus is greater than the WT will ever admit or teach.