Scars in Dragonball are extremely inconsistant. I have no idea if mortal wounds healed via magical means (whether by dragonballs or Dende or senzu beans) leave scars, but I assume not. Krillin was impaled by Frieza yet showed no sign of a scar. Vegeta is hard to tell due to having a CRAPLOAD of scars shown when he showered early in the Android saga (but then they all disappeared later on) but somehow his face is still pristine and pretty. Other than that, I get the impression Saiyans don't scar at all, but humans do, depending when it happened. Like the one on Tien's chest from Tao's knife.
That damn sword causes more plot holes than any other object in the series. It breaks in Movie 7 (Super Android 13), but is somehow repaired in time for Movie 9 (Bojack)... To say nothing of the whole business with Tapion (and/or Minoshia) and those swords which are the same only not the same. To the point, Mirai Trunks' sword was just a normal human sword of unknown history (the only reason he could cut through Frieza was because he put his ki into it, hence why it failed to cut Android 18 [her ki was stronger])... it wasn't a magical item forged by an alien priest, yet are treated by the plot as being the freaking same. To that end, Tapion's sword ends up being used by Trunks in GT (in the opening, yes, but I honestly can't remember him EVER using it in the actual show, so it obviously wasn't very often) which you'd think it would be very useful, being magical and shit...
In any case, being that Mirai Trunks' sword was rather normal, it could have easily been repaired (certainly by Bulma) at some time. Perhaps swords like that are available on every street-corner in the future, and he just got a new one once he returned (Back to The Future!) and trashed the androids and imperfect Cell.
Trunks: "Okay, okay! Relax, dad! It's me, it's me, Trunks!"
Vegeta: "No, it can't be you. I just sent you back to the future!"
Trunks: "Oh, I know you did send me back to the future. But I'm back! I'm back from the future."
Vegeta: (Wait... I'm your father??) "Great Scott!" -faints-