And in the last line , if 'fiosrachaidh' is genitive AND is not linked to 'ionad' by a hyphen, there is no need for an article.
If there was a hyphen, the article would definitely not get put between but there's still an error in
aig an ionad fiosrachaidh.
notices should be short (space) and idiomatic to the language used , i.e. shorter (forget 'nan seirbheisean etc) if necessary
Idiomatic without any doubt. Short is dangerous. Gaelic uses more letters for the same number of speech sounds so it almost invariable needs more space to convey the same "message". It's actually something we need to accept and live with. Yes, if you use idiomatic Gaelic, you can avoid laborious constructions but on the whole, it'll still be longer.
Dropping the article is an interesting questions. Gaelic
likes to use the article more than English does and if we want to be idiomatic, then the article here is most likely called for.
and change register to suit local usage/ taste.
Here, I fundamentally disagree. It's an official sign. It should use a high, official register of the language not specific to the area. You don't get regional fire safety notices in other languages either, both in Bavaria and in Berlin they use standard German. There's no need to that level of regionalism. Besides, what if you use the ferry for a different route, change all the signs?