Redjar:
No. Counter deals a set amount of damage, not determined using the damage formula, so Reflect can't change the amount of damage dealt by Counter. The only exception is if the opponent is immune to the attack.
tarspaceheel:
It can indeed work that way. Fling is a Dark-type attack, so Justified raises the ally's Attack.
Since the item is a Berry with a "when" trigger condition, its effect triggers even if the ally has more than 1/4 of total HP.
My description for Fling reads in part:
"... If this attack is successful, the thrown item takes effect."
"If the item thrown is a Berry with a "when" trigger condition, a White Herb, or a Mental Herb (but not Berry Juice), the opponent uses that item even if ... the item's trigger condition does not hold."
My description for Justified reads:
"Whenever bearer is hit by a damaging Dark-type attack by a non-bearer, bearer's Attack is increased by 1 stage. "
My description for Salac Berry reads:
"When holder's HP is less than or equal to 1/4 of total HP, and holder's Speed stat stage can be changed by this effect, increases holder's Speed by 1 stage and this item is consumed."
(Has a "when" trigger condition, so "increases [opponent]'s Speed by 1 stage" when it's thrown)
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Your second question asks whether the above strategy is viable for Terrakion. There are several points to consider. First, using a damaging attack on a Terrakion, such as Fling, necessarily involves dealing damage to it, thus making Terrakion weaker in future attacks against it. However, Fling with a Salac Berry has a power of 10, a very low power for an attack, so the amount of damage dealt may be negligable. Second, having Terrakion's ally hold a Salac Berry necessarily involves giving up other items such as Leftovers, Focus Sash, or another item that may be more useful to the ally. Third, the combo will work only once, since Salac Berry is consumed when Fling is used. The important thing, as always, is to test this strategy before committing to it.