As for the robot, although powerful I don't think it can make force fields. Maybe something [similar] if it's within the MU, but otherwise not.
We don't have a source showing Mata Nui making a force field, but we have none saying that he can't make them. I'd assume not, though.
In terms of hypothetical battle scenarios, it would make sense to assume not (if you were given the robot for such a scenario you would be wiser to plan for minimum potential strength, just in case it is true), but that's not the case here. Theorywise I'd think he can. He should be able to manifest all the powers he could do inside the robot outside as well. There was no indication in the final battle that the powers we saw him and the Makuta weild are all they have; when the Makuta thinks of using a gravity attack for example, it seems implied that he's just selecting one power out of a wide range of options, rather than using the only option.
The problem is it would be a power drain, so he likely does not generate one often. However, if there is a hyperdrive for the trips between planets, and even if there isn't, a shield is likely generated when he's in deep space, as even with the superstrong substance of his hull, over time impacts from space debris, esp. at high speeds and definitely if there's hyperdrive, would likely take their toll. So would be wiser to use energy shields as a safeguard, and since the Great Beings obsessively think in terms of such safeguards (even giving such shields to two of their important creations, the Bahrag), seems reasonable to theorize they did that for Mata Nui.
But this really has nothing whatsoever to do with his face's shape. It's not like he would actually wear a functioning Kanohi mask, lol. The mask is actually likely symbolic of the camouflage island.
Also, the identity of it as a Hau probably has more to do with the Matoran's perception of Hau-wearers as leaders, possibly stemming back to rumors of the Toa Mata, and seen also with Lhikan, plus an infected Hau was the Makuta's choice of symbol in 2001. So, Hau symbol means "important person", and Mata Nui is included in a subset of that, rather than it meaning the Great Spirit directly.
To the topic question -- if it was a different shape, it would be a different story, but likely not significantly so. I personally feel the mask's shape is not practical for a giant metal thing pushing up through layers of broken-up (by Bohrok) bedrock and earth. The texture of the edges of the eyes, the bottom of the mouth area, and the top of the gap in the forehead would create massive friction as the material slid away, probably doing more damage. Of course, this wouldn't apply for a heavily modified Hau-style shape, and that is in fact kind of what we got with the mouth area, actually. See:
Notice the arch shape around the mouth, and the two slight indentations on the cheekbone areas, similar to the indentations on a Hau there. But the outer edges of these indentations are lessened in thickness by a lot, so much less friction, and the Hau's indentations from the top, bottom, and sides of eyes are removed too. So, the real answer is, "it kinda IS a Hau."
Kinda. Especially more similar to Lhikan's than Tahu's.