Many thanks, Eryniel, for providing this clarification. It thoroughly answers my question about the reconstructed l-forms and also explains another puzzling thing in Pedin Edhellen: the use of
le as a plural pronoun.
However, I am generally reluctant to accept a solution that disregards fully compatible data. If Tolkien really ever thought that
-l- should be the general marker for all the courteous 2nd person forms in Sindarin, it has left remarkably few traces in the sources. On the contrary, he consistently defined the Sindarin
le as a
singular pronoun only*. Pure Sindarin had
de, dhe as the courteous pronoun with the same form for singular and plural. But in Quenya singular and plural differed, and as the Noldor wanted to introduce this distinction in Sindarin too, they borrowed
le from Quenya to serve as the courteous singular. No change was made to the plural; it continued to be
d(h)e in Quenya-influenced Sindarin as well as in pure Sindarin.Thus the connexion of
-d(h)- with the 2nd person was retained, and consequently I can't see any logical need to exchange the attested
-dh- for a hypothetical
-l-.
So: "Kann man schon machen wenn man will" -- ich wage froh den kühnen Lauf

.