I'll address budgetary issues later on ... I am constantly thinking about that in regards to this silly "fan-plan" and thus far I think I've managed to theoretically been able to mitigate a lot of the budgetary increases that would be required for a Dual-PC storyline by removing certain obvious components from DA:I and balancing some of the features out between the two PCs (like LI's and Companions). The only exception that I haven't been able to compensate entirely for is the additional voice work needed for the two voice actors of the new PC, which I'm not going to lie ... is a big one. But again, I'll pitch my ideas in regards to that in a future post.
Seems interesting. I look forward to reading it.
As for the Inquisitor's combat. That is another issue I plan on making a post on, but I have already started it in regards to my "Class Specialization/Unique Character Talent" Post.
Sorry I overlooked it. I will try to go back and find it.
In terms of actual talent trees, there will be at most an additional 4 required to facilitate the Inquisitor functionally as an additional combat capable PC. Clever aesthetics, and perhaps a bit of tweaking in terms of feel of the combat of both character may be required, but again ... later post (this will be the one I touch on my ideas for the prosthetic as well). 
Finally, again this is not a thread advocating for the Inquisitor's return, rather it is one that is meant to focus on the seeing if its even possible mechanically of having them return in a Dual-PC setting. Its focus is on the theoretical and the ideas that people come up with in which they believe they could get it to potentially work, myself included, and "wanting or not wanting" the Inquisitor to return is pretty irrelevant as that was never the point.
Oh it's definitely possible, if that's all you're asking. Here are some of the possible ways I can think of.
1. Unique substitute for all (x)Inquz. Magical arm for non-specs.
2. Get the original arm back. (More on this below.)
3. Rune Golem Arm Gauntlet for all (x)Inquz.
4. Leave the (x)Inquz without the arm and modify (reduce) their combat capabilities. But I don't see rogues picking locks anymore, or at least not without a prosthetic.
Regarding option 2, they could start the game with the (x)Inquz being without the arm, but then the new protag recovers it by going into the Fade. Which means, Solas sent the arm and the anchor to the Fade, perhaps to contain the magic and work on removing the anchor and taking it on himself. So the new protag finds it before Solas and returns it to the (x)Inquz via magic. This would also mean that the (x)Inquz would not have had the arm amputated after the mark was removed. Trespasser does not show what happened, so its still possible to pull this off.
The real question is what to do with the anchor. Even if the arm is returned, it will still kill the (x)Inquz, right? Well, I believe that the (x)Inquz's mission at this time could be to research how to control the anchor. And I think an interesting twist would be to have them find the research done by the only other one who showed control over the mark...
I think it would also be interesting to approach that in multiple ways. You could go through Dorian, Calpernia (if allied)/Samson (if alive), Alexius (if allied), or even Livius Erimond (if made tranquil), or through some other Tevinter contact.
And it would be ironic. Cory's research into the mark and using it to destroy the world is what ends up helping to save it and the (x)Inquz.
That being said, you are right that it would be cheaper to make the Inquisitor an NPC, but personally I certainly expect Bioware not to make the same awful mistakes twice (all new ones, that is perfectly fine ... they've had brand new ones every game. At least they're trying new things). 
They would likely limit the (x)Inquz NPC even more than they did Hawke in this case. The reason Hawke was not well received by most is because she stated her opinions too much during her related quests, and fans forget that Hawke is not our character. She is BioWare's character. I knew this the whole time, and so her portrayal in Inquisition fit my Hawke (a default female archer) just fine.
With DA4, BioWare would likely have the (x)Inquz stick to the missions and just give orders. As if operating off of the Chore Table. The only time the (x)Inquz would express opinion is when controlled by the player for certain missions.