There's a lot to unpack here so I'll split it up. Here we go!
Yes, Tali gave the evidence that got Shepard to be a Specter and that is important. That's one point for you. Unfortunately, that's it.
Her god-mode hacking skills are nice from a gameplay aspect but are never plot relevant. That she is the first to see Shepard is not relevant at all, though I should point out that she is the only one with a natural, believable, and appropriate response to seeing him, though she recovers rather quickly. She might be the admiral of the migrant fleet but that's not only optional because she could be dead, but is just because Shepard's friends get important jobs. Garrus has pull with the Turian hierarchy, Liara is the shadow broker, VS gets promoted and becomes a Specter. Of course Tali is there for the Rannoch arc but that can end with her and the Quarians all dead and the Geth helping the war effort. Her potential to die doesn't necessarily make her totally insignificant, but it does make her less significant and necessarily so because she might not be there! She is only necessary for Geth/Quarian peace, which is not necessary for the plot.
Liara is the one who directs us to Ilos, which is no less important than Tali helping Shepard become a Specter. So she's already tied Tali. Now to fly past her. Between games she recovers Shepard's body, without which there would be no ME2 or ME3. Mind you, this whole thing is stupid, but it's what we have. She leads us to two squadmates in ME2, though either one of the two can be ignored if Tali is recruited. She found the Crucible (yes, it's also stupid, but again, there it is) which is plot relevant. She also has more non-romance scenes with Shepard outside of missions, the two most significant being her Vigil, shown in the Refuse ending, and her mind-meld "gift" at Hammer FOB. She is the only character guaranteed to be there at the end and, like Miranda in ME2, there is only one way to kill her and you pretty much have to do it on purpose.
Really all you've done is arbitrarily bloated the importance of what Tali does and diminished the importance of what Liara does.
This is silly inventing the narrative but I like it as an idea that could have been used. That certainly would have been a better conspiracy by Udina than the stupid Citadel coup.
This is a really good point. They should have actually focused on Garrus being a leader again after his self-labeled "failure" on Omega. Instead his "team" was just some exposition on what he's been doing since ME2.
If you considered all the main characters in ME1 and applied them to the trilogy; each of them have varying degree of importance.
To me, Tali is the major crucial narrating character in the trilogy with a consistent Synthetics-Organics conflict theme in the entire trilogy. Her personal arc starts from being a pilgrim kid who sacrificed everything to reach Citadel because she found incriminating evidence from a Geth that a Turian Spectre went rogue and nearly destroyed a human colony. Why would Tali go such length for a human colony? Especially an alien, Liara, Wrex and Garrus was all indifferent to humanity's plight but she's the only one who risk her life just to help the humans and expose a conspiracy. She could have just send the data to her father and let migrant fleet decide her cause of action. The heretics geth army are her people's responsibility. Her people created the geth and she's the sole person who actually stand up for her people and dealing with the consequences.
In ME2, she's an older and experienced Geth specialist who do more actual scientist thing than Liara; data collection, sample collection. But the Synthetics/Organics theme conflict that drive her narrative again, Geth was involved in her personal conflicts repetitively, we see her grieving for her friends and her family. Shepard's actions by resolving the conflict between her and Legion was at the same time cemented hope that their people could work together.
In ME3, she changed so much in several months. As an Admiral, she pushed for peace with the Geth, considered Legion as her friend even though she still had her own reservation. Once peace was achieved, she became enlightened and had a hopeful outlook for the future. Her people have their home again and they gain a valuable ally and they were instrumental in supporting the war; evacuating the civilians and helping the resistance in reaper occupied territories. Like EDI and Shepard, the Quarians and the Geth are thematic significant, showing that Organics and Synthetics with free will, could work together in peace. Of all squad members, Tali's arcs across the trilogy itself is consistent. I'm not even a Talimancer but seeing the game narrative objectively, she's by far have the most involvement in the greater overarching storyline and that your past decisions do actually matter the most in her arcs.
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I can't say the same thing with Liara. In fact nothing in Liara's narrative in the trilogy took account of your actions to her. Her presence with the confrontation against her mother is not required. Whether you pick her up very last or if didn't speak to her at all in ME1, she still have a big mighty crush on you and she still act like you're her best friend, you still get hugged by her upon meeting, she still become Shadow Broker whether you play LOTSB or not. Her role in retrieving Shepard's body only was acknowledged in the DLC and the comics. Even if you're indifferent to her and in love with another, the game still make it obvious that she had a big gigantic crush on you and the game did punish you especially if you romance someone else because how dare you to ignore her charms. If you do her Shadow Broker fetch quest, you get additional scene with her father where you have a choice to resolve her daddy's issue, which is still irrelevant btw. Post-Thessia, whether you care about the Asari or not, you always lose your cool, not because of Kai Leng taking Vandetta, its because Thessia was destroyed. You're sad and depressed because you promised a certain pretty alien girl that you will save her people and you failed her 
Depending on your choices, you can have Wrex, Ashley, Kaidan, Tali and Garrus all varying degree of dead and neglect but you still have the exact same Liara's narrative in all playthroughs. The only thing that was made remotely important about her in the whole trilogy was how thick her plot armor was. Everyone else get simple dialogue branches and interrupts, she get the special requirement of at least a hundred hours worth of trilogy playthrough to get below 2000 EMS because its the only way to see her plot armor status getting destroyed by Harbinger.
Her LOTSB success and her massive fanbase was the reason why she was given expanded role without any justification of her plot armoring.
In particular, I didn't feel she was that sincere about stopping the reaper and saving the galaxy. She's only interested in it because of Shepard, regardless of her romance status. Her personal motivation was paperthin and frankly, she is the most boring Shadow Broker. Aria, Miranda or even Kasumi could do a better job than her. Hell, she could have bribe the Citadel council and the Salarian and Asari government laundered all their dirty secrets just to gain their cooperation. Instead, the most renegade thing she did in her capacity as a SB was being b*tchy to Jack because Jack barely remember her.
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Officially. 
