Making extremely powerful mundane combatants?

You are right, my mistake. I was confused by the fact that in some cases (e.g. negative sympathies, or sympathies from Faerie Background) Warping Score adds to the initial "free" score, rather than to the limit only. So, I've overspent 45xp (Sympathy starts at 1 rather than at 4, i.e. at 5xp rather than 50). Two easy fixes: either bump up age by 3 years, or reduce a few scores. As I like the "young looking lass who's a terror in combat" concept I'm about to edit the original post to do the latter, reducing from 3 to 2 each of Etiquette, Second Sight and Grant.

You can add one more minor Virtue if you add one more minor Flaw, since there are currently 9 points of each.
However, adding Cautious with Single Weapon seems rather pointless, since it's easy for Yvonne to have a "hanging" Grant that activates, Granting her Cautious with Single Weapon, the next time she is in a real fight (and possibly a second one that activates the next time she is practicing, so she can "rebuild" up her store of hanging effects without ever risking a botch).

I had made a small mistake with the "initial" score of the Sympathy that had caused me to overspend 45xp (i.e. 3 years of experience). I fixed it. Not including the "xp" Yvonne gets from Virtues, such as the initial 5xp in e.g. Second Sight, or the bonus xp from her Affinity, Yvonne has 585xps. Of these, 175 are spent on her Sympathy; 150 on her Single Weapon Ability; the remaining 260 on other Abilities. Note that 260xp is what a "normal" character gets by the age of 14-15, and Yvonne has spent almost 20 years practicing swordsmanship obsessively with her mentor in Faerieland.

Yvonne's very high Sympathy is, mechanically, central to her concept, because (in addition to allowing her to outstrip her "normal" ability limit with the sword) it gives her access to faerie rites of a reasonable magnitude and all the nifty bonuses she can get from them.

Ok, I understand the point of this now. Thank you!

In the case of Faerie Background Warping Score sets the value (for both positive and negative Sympathies), not adds to it. That would put it at 3, not 1+3=4.

You misunderstand. I didn't say "Cautious with Single Weapon." What she really wants is Cautious with Enchantment (Cautious with Grant being the second-best choice). Then she's a lot safer getting Cautious with Grant via Enchantment+Grant, which makes her a whole lot safer getting everything else. This is because your comment "without ever risking a botch" seems to ignore the stress die for the faerie rite.

Yes. Which means that in all the cases I cited, the resulting Sympathy (whether positive or negative) equals:
+ , i.e.:
0 + Warping for Faerie Background
1 + Warping for Faerie Antipathy, Minor
3 + Warping for Faerie Antipathy, Major
That's what confused me!

Meh. Just roll a simple die for her rites and avoid any risk of botch!

The only serious advantage I see when rolling a stress die with Enchantment+Grant or Enchantment+Ware is that you have a chance of rolling a 1, which means your Sympathy will go up without requiring you to spend xp on it. But Yvonne already has a lot of opportunities of doing that by rolling Single Weapon with her Sympathy as a bonus.

I don't think that is allowed. It is vague because the Sympathy Traits apply differently to the rite, but they do apply. We can see that they are affected normally by 0s and 1s as if they apply, as well. When Sympathy Traits are used the die is always a stress die. Though far from explicit, I think the intent that this roll must always be a stress die is pretty strongly implied.

RoP:F explicitly states that if you know Enchantment, you can always choose to roll a simple die rather than a stress die on the Casting roll (even though a rite always requires Sympathy):

Ah, I'd missed that. Of course, it seems to pretty-strongly imply the extra rolls it requires must be stress rolls. Can this character reliably perform to get that high without Sympathy Traits?

Uhm, with "extra rolls" do you mean the Characteristic + Ability rolls (in the example given, Dexterity + Single Weapon) that one must perform before rolling Enchantment + ? If so, those are covered by Cautious with Single Weapon. Stress dice, yes, but with two fewer botch dice, which means no botch dice at all unless Yvonne trains or fights in truly horrid conditions.