Go raibh maith agat!
C-4: Níl an briathar 'bainim le' i gluais an leabhair so ach is dóigh liom, mhuise, go bhfuil an ceart agat.
'beirim ar' is of course 'seize' (etc.), but this is the closest equivalent I've found in the book.
CI-3: aon bheirt: use of aon with the plural always trips me up.
Would this work (to use deallrathach/deallramtach/dealraitheach that is given in the examples in Lesson CIV)?
Ní fheaca riamh aon bheirt chómh deallrathach lena chéile le Tomás agus a athair.
I must admit that those two 'le's look a bit weird.
Also: are 'lena chéile' and 'le chéile' interchangeable here?
Corpas Stairiúil gave me:
É sin agus Cnoc Tuirc cois Léin-Locha, táid siad deallrathach len a chéile ar an nós san. (Tíreóluidhe ar Éirinn - Tóibín, Seán)
Tá siad chómh deallrathach le chéile is dá mbainfeá an ceann de dhuine aca is é chur ar an nduine eile. (Mac Cárthaigh Mór - Ó Séaghdha, Pádraig)
Yes. I've been using a different scan available at
https://archive.org/details/cu31924026508022, but the one you linked seems to be an exact reprint (with the dates strangely missing). The key has been quite workable so far. It's only these two exercises that don't have complete answers.