When conquering a battlefield during an opponent's turn while already controlling another battlefield from a previous turn, does the previously held battlefield count toward satisfying the "scored every battlefield this turn" requirement for the 8th point?

No — a hold from a previous turn happened in a previous turn, so it does not satisfy the "scored every battlefield this turn" requirement for a Conquer-based Final Point.

Scoring is evaluated per turn (Rules 469–470): holding happens during your Beginning Phase, so a battlefield you held last turn was scored last turn, not this turn. If you're at 7 and conquer the other battlefield during the opponent's turn, you have not scored every battlefield this turn — you draw a card instead of gaining the 8th point (Rule 471.1.b.1). This matches FAQ 3948: you'd need to also score the battlefield you already controlled during that same turn (e.g., by re-conquering it after losing control) before the Conquer can win the game.