Is delaying going to Friday prayer—with the intention of skipping the innovated pre-prayer Sunnah that the mosque's imam stipulates—worthy of reward or accountability?

There is no Sunnah prayer before Jumu'ah. If you delay until after the second Adhan and the Khatib has ascended the minbar, you have missed a great virtue. However, if you enter the mosque after the first Adhan and pray Tahiyyat al-Masjid (greeting the mosque), it is better for you to go early to Jumu'ah and perform as many voluntary prayers as Allah wills for you before the Imam ascends the minbar, based on the Prophet's (peace be upon him) saying: "If a man takes a bath on Friday and purifies himself as much as he can, and puts oil (on his hair) or scents himself with the perfume of his house, then proceeds (for the Jumu'ah prayer) and does not separate two persons (sitting together), and then prays as much as Allah has ordained for him, and then remains silent when the Imam speaks, his sins between that Friday and the next Friday will be forgiven." You can also search for a mosque where this transgression is not committed, and advise the Imam with gentleness and kindness.