Let me explain what I mean by this. We simply replace Artoria with one of them. We keep the main events of the Fourth Holy Grail War the same, but change the outcome. That is, whether any of them would come into conflict with Kiritsugu or not. And who would be able to handle the six enemy Masters.

Nobody.

Gilgamesh is still there, and all of them are much weaker than him (and unlike what happened with Saber (Artoria), he is not going to hold himself back against any of them). Moreover, Siegfried has a major weak point that Gilgamesh can use against him (and Gilgamesh is aware of that weak point). As for the others, they're still much weaker than Gilgamesh.

By the way, who is stronger — Arthur Pendragon (as Saber), or Artoria (as Saber)?

If you mean the relationship with Kiritsugu, I think it would be like this, more or less:

Arthur would act more or less the same way as Saber (Artoria) does.

They're not that different in their personality (at least as I understand it).

Siegfried would follow orders even if he doesn't agree with them.

And I don't know enough about the others.

Altera would be interesting case... I mean, she can't beat Gil, at least without Master's support but she could have beat him if she turned into Sefar, but that's only Moon Altera.

Earth Altera meanwhile can't do it simply due to how Moon Timeline and Earth Timeline go (aka Sefar's real body not going to the Moon Cell in the latter due to it not existng). I mean she is still strong, matching Iskandar in Fate/EXTELLA (I mean it's Moon Altera, she beat him in their first fight but only due to the former not being used to the Moon Cell but in rematch, they were not only seemingly equal and had a draw. I could probably try to use Fate/EXTELLA Link info too... if I had played that game fully which I didn't. Anyway back to Earth one since we are focused about her and not sure if Moon could scale Earth Altera which is probably no.)

As for if Kiritsugu had any conflict with Altera. Well, it says that, quote:

"She treats her Master stoically, recognizing them as a general or adviser-like figure, and will generally go along with anything they say with an “I see”. Instead of trusting as a human being, the situation is closer to a fighting machine on the battlefield that fully relies on external software (the Master) to process thoughts."

So she would probably be okay with him. And if needing to destroy the grail, she wouldn't mind it either, despite having a wish.

Probably only issue would be Gilgamesh being in the war but there may be a chance they could beat him if Gil decides to go a bit soft (which he will probably don't) but you know what else he could do? Maybe use the favor Gil has with Altera/Sefar for sparing Gil's father since with it, that's how Gil joined Altera faction in EXTELLA. I mean it may probably don't work since Gil only kinda truly joined to make sure Hakuno was safe but still.

If that will not work, then I guess they must pray that maybe Altera can use Teardrop Photon Ray (but it will probably not work but eh, they can try).

Would Kiritsugu have supported her?

You mean the mana for her to use Teardrop Photon Ray? Probably. It says it cost a great amount of mana and since Kiritsugu didn't had a good training and is average as a mage (according to his page at least), I don't think he would (unless Irisviel will give her mana to Altera then maybe but I don't think that's possible).

But if you meant as Master-Servant duo? Well, Altera has a thing that thanks to being now a Servant, she can feel emotions but usually doesn't know how to express them and feeling uncomfortable with them. She also basically lacks experience in everything besides fighting and war, so maybe it will remind Kiritsugu about Irisviel due to the similiarities. So I would say their bond would be probably akin to one he has with Isriviel, except probably not romantic. Heck, I could see Irisviel trying to bond with Altera about those similiarities.

In short, in mana for Teardrop Photon Ray if you meant that. No, Kiritsugu would probably died from it, I think since we aren't sure how much mana was needed but considering it's probably a lot that average magus can't have, then yeah.

As in Master-Servant duo, they will do probably well. Not only due to the similarities they share but also due to the ones that Altera has with Irisviel and probably Maiya a bit too. Though since I didn't watched Fate/Zero, I can't tell exactly if Im correct. (EDIT: Though Altera's urge to destroy may be a bit problematic since it's described as something more powerful than use power of the command seal. So yea, if Altera goes a bit too destructive into it, yeah.)