The first lines of the lute player's promised song (Doch trinkt noch nicht, erst sing’ ich euch ein Lied! / Don't drink yet, first I’ll sing you a song!) are a mirror image of the last three lines of the Abschied, complete even, with the celebrated '[E]wig':
Das Firmament blaut ewig, und die Erde
Wird lange fest steh’n und aufblüh’n im Lenz.
This suggests to me a bookending of the symphony, and this seems to put a premium on the lute player's song. At the climax of the lute player's song, the command, "Now! Drink the wine!" rings out to a gathering that is witnessing a diabolic spectacle.
On its own (ie., without the music), the text suggests to me that when we see evil violating our world, we must accept it. The worst disasters of life are an inescapable reality, and sometimes, are even the point (hence, 'Now! Drink the wine!').
Well that's fine as far as it goes. But the music seems to me to go even further. Mahler's score settles with surprising haste from this turbulence and horror, into the dreamy and peaceful passage at "[...] Becher zu Grund! Dunkel ist das Leben, ist der Tod".
To accept the reality of evil (or disaster, or the occult, or whatever is conjoured up by the image of the howling ape) is one thing, but to dreamily embrace it is quite another!
This easeful sharing of space with the screaming ape doesn't seem to be reinforced in the Abschied. Why does Mahler want us to be so positively relaxed with it?