Pretty much that. There is a lot of wiggle room. It doesn't matter too much what you do apart from getting heighten and transmutation foci. I like evo for slightly better stormbolts. You'd also need pretty much sorc/AT gear and would want amulet of four elements and arcane annihilation robe once you start stormbolting.

Doesn't using disintegrate (if it gets the final blow) screw you out of the loot that the person would drop though? Is there any way around that?

I guess another question is, how bad is she at her job compared to say, an AT sorc like in your main post or a DC focused Sage sorc? Because if it's that bad, I'll probably just hire a merc and copy one of those builds or use the respec mod to put her in one of them. I don't like "cheating" but I want to play on a difficult setting while not being gimped too hard by arbitrary companion class choices.

transmutation is there not for disintegrate, but for slow. You will use hellfire rays with rod of flaming vengeance late anyways.
she is an 8 con wizard that can't even use profane staff from varn. How bad do you think she is? It's possible to beat unfair with her and that's that. She has no redeeming qualities aside from being a companion.

Forget about Disintegrate, save her 6th spell slots for Hellfire Ray and Tar Pool.

Also, if you ask me: how bad Octavia compared to a Arcane Trickster mercenary?
My answer is: just as bad as Jubilost compared to a Grenadier mercenary, which means she's good enough.

Cheers boys, I think I know what must be done, lmao.

jubi Has redeeming qualities.
a) his stats are at least somewhat sensible and are not made with the sole purpose of annoying the player with constatnt bitching and moaning about rest or constant amulet swapping.
b)Jubi has the best camping ability in the game. roughly 10% camping efficiency is well worth slightly less optimal alchemist.
Octavia does not have anything going for her. I can't name one thing she does better than a merc. She could be of use if she'd let jaethal to... improve her. Now then she'd be rather cool. That'd swap her to evil and give her the ability to use the staff and it'd let her use her otherwise useless cha for hp...

What I did is I set it to unfair and then jacked the difficulty up a bit more , but I used to respec mod to make tristian the erastil cleric, octavia into a Sorc/AT, and Jubilost into a Grenadier. It's a lot more fun, and it's still as ungodly hard, especially since I compensated for the optimization. The guide is doing wonders, cheers!

glad it's working out for you.

On the topic of the arcane archer: It seems that you don't suffer from arcane spell failure from light armor for any arcane spells (even if they are from another class) if you have level of magus. Must be a bug right?

not sure. I've tested the damage output in the HatEoT and the ability to ohe-shot stuff on unfair if going nova and that's pretty much it. Could be the case. I'm not huge on archers in most any game.

According to description spell combat shouldn't work the way it does either. Believing what the game tells you is the last thing you should do with this one.

Is there much I need to change other than weapon focus/etc to make the Melee sorc build work with ovirnbaane? (using the non ovirn build on valerie)

You'd probably want to be 18/16/13/7/7/16 human for that. Other than that - no, not really. With an added feat everything will fall in line.