You only have to look across evangelicalism today and see the WIDELY and VARIOUSLY held views of the Bible and KNOW that there is no unity at all.
Dispensationalists are Christian in the same sense that Moslems or Mormons are Christian. Both Moslems and Mormons hold the New Testament as relevant to their respective canons, but their surrounding scriptures change the meanings and force contradictions with the New Testament. For Mormons, they change certain intended meanings of scripture to suit their ideology, including changing the established context of "The Word was God" into "The Word is a god". Moslems accept scripture as a deuterocanon but claim that it has been corrupted (the claim for imperfection addresses contradictions in that case). In Moslem scripture, Jesus talks to God the Father and states that he never told the people to worship him as God. Dispensationalists as well reject the clear and straightforward meaning of different passages from Gal 3, Rom 9, and other scattered verses to suit a hybrid system between modern Talmudic Judaism and a subservient goyim slave race of authorized servants (which are required to fulfil the Talmudic version of the day of the Lord). Dispensationalism is as different to Christianity as Islam is to Christianity. So yes, if you believe in Dispensationalism, you are part of a different religion, with a different body of hermenuetics than what is possible within Christianity.
The objection that "I believe that Jesus is the Messiah! I read the NT in an edifying way" is a statement that is true for Moslems as well. Being a Christian is more than just accepting Jesus as the Messiah and reading from the NT. There is a foundational difference between these different groups.
If a Talmudic Jew came up to a Christian and stated, "My people are the rightful owners of this land, because God said so, and denying that is to call God a liar!" From their perspective, they are being honest to their sincerely held beliefs. And that's fine, because they are of a different religion and Christians shouldn't feel obligated to necessarily hold them up to a high standard. It's no different than an atheist rejecting God, etc. They'll come around eventually if they are meant to. But denying the Talmudic interpretations of the land promises does not mean that Christians would be calling God a liar. Christians are following the truth of the Holy Bible when it states that Christ is the seed of the promises and that those of the flesh are not children of God. Dispensationalism resonates more with Talmudic Judaism on this topic and from that is a Talmudic religion in that sense, taking many points, philosophies, and interpretations from a perspective in line with Talmudic Judaism. It's not that your claims would necessarily be inconsistent with the source material you hold in authority, it's just that those materials differ from the established canon of Christianity. This is no different than the added books from Joseph Smith for LDS or the Qur'an and Haddith for Islam. Dispensationalism follows some oral authorities that won't necessarily appear in a text but nonetheless change the hermenuetic.
It would be a mistake to call someone else insincere simply because they follow a different religion with a different established ethos. But when two religions are very similar it isn't always readily apparent that they are different religions, and from that it can create the appearance of division and insincerity when none of that in fact exists. It's not that one side is being "stupid", it's just that two completely different sets of belief come from two different sets of foundational scriptures, philosophies, etc. Such is the case when it comes down to Christianity and Dispensationalism.
Your particular brand of Dispensationalism beliefs are unusual because you reject the standard Dispensationalist position that hypergrace would exist for all Talmudic Jews. Dispensationalists generally interpret Rom 11 to mean that all Talmudic Jews would be saved. Dispensationalism espouses the belief that all nonJews are second-class citizens. The trend I have seen in many cases is that many Dispensationalists themselves will have some kind of Talmudic Jewish ancestry that in the back of their mind will make them feel comfortable by seeing themselves as some kind of upper-second-class or lower-first-class. Especially if they are married-in and/or have children with an established ancestry. It's the same kind of mentality as a poor person believing that they aren't really poor, just temporarily embarassed millionnaires. The pyschological appeal makes sense in a lot of cases, especially if a person had Talmudic Jewish relatives or loved ones that they care about. To even considered the idea that a loved one that has passed away could have missed the opportunity for salvation is unbearable. So if someone had a deceased Talmudic Jewish grandmother that they loved, having someone tell them that none come to the Father except through Christ would probably feel evisceral like a knife in the gut. It would be much more psychologically comfortable to invest in the idea that salvation was theirs by birthright rather than accept that God ultimately has the choice on who to show mercy to.
There's a lot of angst that can come from this topic. Sometimes challenging people's false beliefs will make them react as though you've slapped their newborn baby in the face.
Emotions can cloud sound judgement. Emotions can distract from the ability to think with sober logical thought. If someone's reaction is extreme to the point that they act like you've slapped their newborn baby, they very likely have a deeply rooted emotional investment.
If someone doesn't have those visceral emotional reactions, there is a better chance at appealing to their sense of reasoning. And for those Dispensationalists that aren't ruled by emotion (if there are any) these have the chance of having a reasonable conversation. If you want to have a reasonable conversation between Dispensationalists and Christians, the first place to start needs to be an evaluation of the differences in ethos. That is, the differences in the foundations of the hermeneutics. If you can't acknowledge that Christians and Dispensationalists use terms differently, no conversation will be fruitful. These are two different religions and that needs to be acknowledged first in order to bridge that language barrier. You can choose not to communicate, but the offer is open to any Dispensationalist that is willing to speak patiently with logic.
Even between Talmudic Jews and Christians, maybe by the end of the day there is no agreement, but it is still possible to have a mutual understanding in respectful disagreement. The same is true for Dispensationalists and Christians.