It was that your servant, not to fail the appointed tryst, took the form of a fiend, seized a famous mountain, spirited her to a cave-dwelling, and was matched with her as husband and wife for thirteen years.

不負前期, bù fù qián qī, "not to fail a prior appointment" — he presents thirteen years of abduction as keeping a promise. 攝, shè, to spirit away by supernatural force; 配夫妻, pèi fū qī, to be matched as husband and wife. The verb 配 is the one used of arranged marriages, and of alchemical pairings.

'Not a sip nor a peck but is foreordained.'

And here is the maxim that settles everything. 一飲一啄，莫非前定, yì yǐn yì zhuó, mò fēi qián dìng — "not a sip nor a peck but is fixed beforehand." The image is a bird at its feed: every mouthful allotted in advance. It comes into the language from Zhuāngzǐ by way of Chan Buddhism, and it is still the Chinese for "everything happens for a reason" — with rather more iron in it.

Now the Great Sage Sun has come and accomplished the matter."

成功, chéng gōng, to bring the business to its conclusion. He credits Wukong with completing what fate had scheduled, which is a graceful way of admitting nothing.

The Jade Emperor, hearing this, took back the golden tablet and demoted him to the Tushita Palace to tend the fire for Lord Lao the Supreme — kept on his stipend but under assignment: with merit, to be restored to office; without, to have his punishment made the heavier.

The sentence is pure personnel administration. 兜率宮, Dōushuài Gōng, is the Tuṣita Palace, where Lord Lao — Laozi deified — keeps the furnace in which Wukong himself was once cooked for forty-nine days. The demon is sent to stoke it. 帶俸差操, dài fèng chāi cāo, kept on the payroll but placed on assignment; 重加其罪, zhòng jiā qí zuì, to increase the penalty, with 重 read zhòng. Merit restores him, failure costs him more: a performance review, with fire.