Well, I would argue that leaving the item boxes in is part of how Accelerate’s singleplayer works; without it time trials would likely be quite dreadful. Doing all sorts of crazy boost chains off a pack of triple melons is quite a bit more difficult than simply drifting around corners as tightly as possible.
You can’t pause singleplayer matches in TU. Doesn’t matter what you’re doing; condos, gameworlds, plaza, etc. So even if you press ESC in singleplayer whilst playing Accelerate, the game will continue onwards as normal. Which more than likely means you’ll be losing a lot of time.