The question is, for how much longer have the South Slavs operated the LM? That is which was the real terminus ante quem of this linguistic process?

There are some nonmetathesized forms attested in OCS texts:
Rick Derksen: Notes on the slavic metathesis of liquids

Frederik Kortlandt mentiones two of those here:

"The absence of metathesis in OCS. алкати ‘starve’ and алдии ‘boat’
does not prove that the metathesis was a recent development: it simply
did not reach some of the Bulgarian dialects."

Frederik Kortlandt: Early dialectal diversity in South Slavic I  – on page 3

So not only that liquid metathis didn't reach its terminus ante quem
back then, it was still spreading through Slavic dialects.

The first reference to Albanians as historical people comes from an Old Bulgarian document written around 1010 AD possibly in Ohrid, where the "Arbanasi" are mentioned as one of the "half-believers" (Christians but not Orthodox):

However, in a Serbian document dating around 1200 AD, the region Arbanon is called Raban showing south slavic metathesis

It would guess that those two aren't borrowings from the same time.
"The life of Stefan Nemanja" actually quotes some passages from Nemanja's
charter to Hilandar written between 1198-1999 AD, in it we read:

"… а одь Арьбанась…"; 'and from the Albanians'

[size=8pt]Franc Miklošič: Monumenta Serbica, Vienna 1858, 4[/size]

But in order to go from *dolbok to dlabok we need the south slavic Liquid Metathesis. This means that the LM in this example has occured much later than 1000 AD.

Not sure about this, every syllabic ḷ should have underwent the ḷ > ol > la change if that was the case.
What is the datation of Macedonian ḷ > ol change?