Is he who is martyred in Jihad, and who has missed prayers that he has not made up, sinful, and will he be held accountable for them, and which is better: making up missed prayers before Jihad or during it?

If Jihad is indeed legislated, then it is permissible to go out for it while making up for missed prayers, especially if the Muslim does not experience hardship in their body or wealth. This is more preferable than making up for prayers according to those who do not obligate making up missed prayers, like Ibn Taymiyyah.