> 12.  Téit Nerae íarum coa mnaí isa síd ocus ferais a ben
> fáilte fris.  "Eirc ass trá don dún," ol a ben fri Nerae,
> "ocus beir cúail condaid latt.  At·táu-sa blíadain lláin
> oc[c]a aithigid co cúail chondaid formo muin cach dia tart
> éisi, ocus as·ruburt [is / as] i ngalur ro·bá-su.

Theoretically both "is" and "as" are possible. Since the MS has "is" (IIRC), we can stick to it.

> Ocus uind-[siu / se] do macc sund ucut dano."

Schumacher (KP 382) writes that "-siu" (2nd sg emph. pronoun) must have been the original form (and is attested as such), since "uind" was originally a 2nd sg. verbal
form. But the form was then reinterpreted as a demonstrative particle, and consequently the "-s-"-element was reinterpreted as a suffixed demonstrative particle "-se". If we
want to create an idealised OIr. text, we should emend "uind-siu".

Téit-sium ass
> íarum don dún ocus beirid cúail condaid leiss fora muin.
> "Fó!  Día do bethu assin galur hi·rraba!  Olc lemm immurgu
> in ben do f*eis[s] latt cen athchomarc."

What was our decision regarding "s"? Write "ss" everywhere?

> 13.  Beirid in M[o/ó]rríga[i]n

The "mor/mór"-bit is a matter of personal choice regarding the etymology. As to the ending, etymology on the one hand, and early attestations like Thes. i 2.6 suggest that
the î-stem was original, and that the later inflections as â- and n-stem are analogical. I strongly vote for normalising to "M[o/ó]rrígain".

> íarum boin a maicc-sium céin
> [m]boí[e]-sium ina chotlud, conda·rodart in Donn Cúailnge
> tair hi Cúailngi.

Nearly all examples cited in DIL C 180.18 show a nasalising relative clause after "céin". But then, this may again be one of those cases Thurneysen speaks of in GOI 319:
"a nasalizing relative clause can be replaced by a formally independent clause in almost every instance, even after conjunctions like (h)óre, amal, fo bíth etc." I'd say: "in
dubio pro manuscripto".

> Do·tét cona boin a frithisi anair.
> [Nos·tairthenn / dos·(n-)airret] Cú Chulainn i mMuig
> Muirthemni oc tuidecht tar ais.

We cannot possibly leave "·tairthenn" in an idealised OIr. text. Here I strongly vote for the emendation to class. OIr. "dos·(n)-airret"; the alternative is to take *tairrthid as a
simple verb, but put the right ending "nos·tairrith" on it. But the latter is only a weak compromise, and emending to OIr. is better here.

Ar ba de geis[s]ib Con
> Culainn, † cet[t]eit ban † [cet éit ban?] asa thír, manip
> [aurderg / airdirc / ardarc] leiss.

I'll leave the question "ss" vs. "s" to others to decide. I won't tackle "cetteit ban" and "aurderg" right now.

> Ba dia geis[s]ib íasc i n-inberaib, mani·tuititis [?]
> leiss.

Also no comment on "·tuititis" at the moment.

> Do·airret
> Cú Chulainn in Morrígain cona boin ocus as·bert:
> "Ní·b[e/é]rthar ind immerge!" ol Cú Chulainn.

It seems to me that an order in the subjunctive mood "ní·berthar" makes better sense here.