1) It appears that an additional Boost die when facing a single opponent in Engaged range by itself is considered too powerful; otherwise it would already be written this way.
However, It is also entirely possible you are correct and through initial testing they found it was a wee overpowering.
In this case I'd prefer it flip to one of Dono's suggestions: DT removes a Setback when engaged with a solo foe*. Perhaps even becoming Ranked and replacing Defensive Training in the Tree.
In this way the removal of multiple Setback when engaging a solo opponent would help to carrot the Narrative more positively, without feeling like a lodestone on the character sheet.
* As not every solo foe is going to have Defenses, not every target will offer Setback Dice, however the more powerful and the more important "set piece" encounters will.
2) A penalty that occurs irregardless of the Character's action is inappropriate for a Talent. No talents do this; I'm of the opinion that none should, because this breaks the implied contract, "When you spend XP, you are enhancing your character."
3) Ideally, the Talent offers a character an option, such as:
3a) "When engaged with a single opponent, you may..."
3b) "When engaged with a single opponent, gain..., but when engaged with multiple opponents, you may instead suffer X penalty for Y benefit."
3c) "When engaged with a single opponent, you may gain..., but if you do, suffer penalty X for time Y"