Grand Impact Ready lasts 30s. You can Accelerate a spell and hold Grand Impact until after Fleche/ContreSixte. Just don't Accelerate too early leaving yourself with no procs, which would force Grand Impact.
You can never get two full six step combos under Embolden because each six step combo takes about 12s to execute and Embolden is only 20s. The best you can do is 6+3 or 3+6, so if it can be helped you want to Embolden before the Verfinisher so you get two full spell combos and one melee combo, but you shouldn't purposefully delay anything to line it up like this.
This reasoning is incorrect. You cannot consider only the first cast of the next Dualcast because Dualcast spells are a single construct spanning 5s. It doesn't matter that you still have to cast a 320p spell after using an Accelerated spell.
Potency of an Accelerated spell: 400 p / 2.5s = 160 p/s
Without Acceleration: (320 p + 390 p) / 5.0 s = 142 p/s
Already an Accelerated spell is better than a Dualcast. (Note I have included the proc values in the Verslowspell potency: +20p for an Accelerated spell that guarantees you get a Verfastspell spell to use instead of Jolt, and 10p without acceleration because there is a 50% chance to get a Verfastspell instead of Jolt. This potency belongs to the Verslowspell, not the Verfastspell, because the Verfastspell is only a Jolt if there is no Verslowspell before it, so calculations should be done with all Jolts.)
If you want to consider the next spell after the Accelerated spell you must consider the entire Dualcast:
Acceleration + Dualcast: (400 p + 320 p + 390 p) / 7.5s = 148 p/s
Even when you consider all the spells that you would cast in the 55s between accelerations, it's still a gain. However, it's just not worth dropping a free movement GCD for that gain if using Acceleration on cooldown would prevent you from comfortably handling a mechanic later.
(This post uses current EW potencies, but the mathematical concepts would remain the same in DT)