The Great Sage, seeing that the elder — three times over, back and forth — would not change his heart, had no help for it and made to go.

三番兩復 — three times over and twice back — is a set phrase for something that has gone round and round, and it is the narrator's summary of the whole afternoon. 沒奈何才去: only when there was nothing else to be done did he go. He has not stormed off. He has run out of options. (那 points, nà; 長老 takes 長 as zhǎng.)

Look at him:

Three characters, and they mean the poetry is starting. 你看他 — look at him — is the storyteller turning the audience's head toward what comes next.

Holding back tears, he knocked his head in farewell to the elder; swallowing grief, with lingering care he charged Sha Monk.

噙淚 is holding tears in the eye without letting them fall, which is a precise and rather devastating way to describe somebody keeping their composure. And look at what he does with the last of it: one bow to the master who would not accept a bow, and one more instruction for Sha Monk. Even the verse cannot get him to think about himself. (長老 takes 長 as zhǎng.)

With a shake of his head he brushed against the grass on the slope; with both feet he kicked and overturned the vines on the ground.

The couplet is all physical force — the grass on the slope brushed flat, the vines on the ground kicked over — and none of it is aimed at anything. This is what leaving looks like when the person leaving is strong enough to level a mountain and has nothing to hit.

Up to Heaven, down to Earth, as swift as a turning wheel; crossing seas, soaring mountains — first in all the world for that.

And then the poem does something odd and rather moving: it changes the subject to how fast he is. Up to heaven and down to earth like a spinning wheel, over seas and mountains, 第一能, first in the world at it. All that power, and none of it is any use for the only thing he wants.

In a moment his shadow was gone; in a flash he sped back along the old road home.

頃刻 and 霎時 are both "in an instant", stacked one after the other. He is gone before anyone can look up. And the road he takes is 舊途程, the old route — the way he came, travelled backwards, alone. (間 is jiān.)