I have looked at the context...and made a little headway in what's going on.
You're right about the verb, more or less. Although, there are a few thoughts I'd add. One, if you were willing to alter the Ni, the bi could be a command. But even if it is the conseutudinal present, it could be a general statement containing in effect some of the force of "should" (whether or not the you is dropped).
I'm pretty sure that I have seen a nach as a variation of nad, so that could be kept.
But either way that it's translated so far sounds like a copula, so we're not left better either way that we look at the verb (as far as I can tell).
As far as cuitech, it occurs to me that the translation of ALI saw cuit - share, and -ech (personal suffix), and added an -er, even though that rule isn't the type of person involved. This may be something more like "person who has a share" which would lead to a wealthy person eventually (although the commentary on the passage does not seem to agree with the latter translation, as best I can tell so far. (For "he is not a spender that is not a person-possessing-a-share," however you want to read that).
Anyways, without (much) further ado, the commentary upon this passage. The passage it self comes from Bretha Eitgid "judgments of inadvertance," which Kelly says deals with accidental death or injury where no liability is attached. The ms. tradition for the text is somewhat scattered, but I've managed to find the two instances of it in CIH, 323.18, and 1160.38. I'll give for now the first paragraph of the commentary (because I'm not going to try to translate any more tonight.) As the base text I'll give that at 323.18 ff, with "significant" variations below (skipping most variations in spelling, the text at 1160.38 seems to be modernized. My translation is choppy and rushed at best.
NIBI CAITHECH (1) NACH CUITECH (2) i. IN set coitcenn soraindi (3), (4) slan don duine a chuit fein do caithem de, cid re deithberus cid re indeithberus, cia robi(5) in fer aile i baile i roised (6) im cocar (6) cenco raibi, (7) set aotjgom omeocj os esbadach trena roind a hic do risin fer aile (7); set na caithea (8) cuitig in fir aili (9); 7 da caithea, is a aisiuc uad co lanfiachaib (10) gaiti; set munbu (11) duine dligus errech do gabail da cheile he, 7 mas (12) ed, islan (13) do a beith (14) aici re re in errig.
3. sorainne. But further on the text talks about doraindi/doroinde apparently as a contrast.
6. missing this phrase, having "a fiarfaige" (initial f with dot)
7. missing this phrase
9. adds "de"
13. is slan, which is probably the intended reading here, as well
Nibi Caithech nach Cuitech. That is, the chattel owned-in-common
of good-(?) (1), his own share (is) immune to the man to
spending of it (it is immune for the man to spend his own share), though it is with (?) blamelessness, though it is with
non-blamelessness,(2) though the other (one?) man may be (3) in a place in which he may reach about consultation
(cenco?) it is (?). Chattel of restitution of honor, it is deficient through the
division from paying back (to?) the other man; set of the (?) (4)(?) from (?)
the share of the other man, and two (?), it is restoration from
it/him with full-fines of theft (5). (It is a?) Chattel if he is not a man that is entitled (?)
eraic from taking of two companions (of them?), if it is it, it is immune to
him its being with him (?) before (6) the eraic.
1. I'm attempted to take this as something related to rann, division, especially since we are looking at cuit, share, but I am unable to figure out how, or what else is likely.
2. i.e. whether or not there is he/it is blameless.
3. What is the implication of the ro consuetudinal...if that is what this is.
4. The best I found in DIL is trespass, but considering it's likeness to caithech and caithem, this probably has to do with spending
5. Perhaps this should be read as an overreaching when someone spends not only his share, but another's share.
6. Reading re re as dittography in a ms. ancestral to both copies.