Learn Finnish: Ostimme asunnon upeine merinäköaloineen. - We bought the apartment with its wonderful sea views.

In the comitative case, adjectives modifying the noun take the -ine suffix but do not take a possessive suffix. Only the head noun (merinäköaloineen) takes both the comitative -ine- and the possessive suffix (here, -en).

How exactly is the word merinäköaloineen built?

It comes from merinäköala (sea view). You use the plural stem merinäköalo-, add the comitative suffix -ine-, and finally add the third-person possessive suffix -en. The comitative case always requires a possessive suffix on the head noun.

Because it uses the plural stem, does this strictly mean multiple sea views?

Not necessarily. The comitative case is practically always formed using the plural stem, so it looks plural in Finnish even if it refers to a single thing (like one continuous view). You have to rely on context to know if the real-world meaning is singular or plural.

Could I just say upean merinäköalan kanssa instead?

Yes, and in everyday spoken Finnish, you almost certainly would! The postposition kanssa (with) used with the genitive case is much more common. The comitative case is quite formal and is mostly used in written language, exactly like this sentence you might see in a real estate brochure.