It's Late Latin. From the OED:
Originally this governed a following word in the genitive, but in late Latin the tendency to use the phrase as a compound noun appears in vicequæstor (equivalent to prōquæstor of analogous origin). In medieval Latin such formations became common, as vicecomes, -consul, -decanus, -dominus, -princeps, -rector, -rex, etc.
The only thing you're missing is ūnā, which here is an adverb meaning "together". See L&S, part C of the entry. As for the second sentence, yes, in a multi-sentence passage of indirect discourse it's normal for the accusative and infinitive construction to continue throughout, without a repeated verb of speaking.
For reference, the full clause is:
Postridie Nonas Decembres (6.12.2017) anniversarium independentiae Finnorum solito maiore pompa celebratum est.
maiore is feminine ablative singular and modifies pompa (= pompā).
solito is a neuter singular ablative (of comparison), and provides the second term in the comparison. There's the usual amount of pomp, but ...
I think your translation is very good and you have taken no unnecessary liberties.
The only bit of information that is missing is the superlative.
One other small improvement could be made by translating dative + gerundive + form of esse as "we must find", which is, I believe, the standard translation.
My reading of the Lewis and Short entry is that constitutio meant an imperial edict, and over time, came to mean any law. The excerpted text of Justinian says:
quodcumque ergo imperator per epistulam promulgavit, vel cognoscens decrevit, vel edicto praecepit, legem esse constat; hae sunt quae constitutiones appellantur.
The Online Etymology Dictionary ...