Learn Italian: Il messaggio è ambiguo, devi scrutarlo attentamente. - The message is ambiguous, you must scrutinize it carefully.

Even though the message itself is uncertain, the speaker is stating a definite fact about it: it is ambiguous (è ambiguo). The subjunctive is triggered by specific phrases expressing doubt or negative certainty (such as Non è certo che il messaggio sia...), rather than by adjectives that simply describe something as doubtful.

Why is the pronoun lo attached to the end of scrutarlo instead of going before the verb?

When an infinitive follows a modal verb like dovere (here conjugated as devi), you have two choices for pronoun placement. You can either attach it to the dropped final 'e' of the infinitive (devi scrutarlo) or place it before the conjugated modal verb (lo devi scrutare). Both are perfectly natural and mean the same thing.

Could I say con attenzione instead of attentamente?

Yes, absolutely. In Italian, it is very common to replace an adverb ending in -mente with con plus the corresponding noun. Saying devi scrutarlo con attenzione means exactly the same thing and is a great way to vary your vocabulary.

Is scrutare common in daily conversation, or is it very formal?