watasi ha kuukou de hikouki ni noruno ga sukosi kowai kamo siremasen.

The particle は marks the topic of the sentence (“As for me…”), not necessarily the subject. Here, you’re setting 私 (“I”) as the topic, and the real grammatical subject of 怖い is the nominalized clause 乗るの (marked by が).

That の is a nominalizer. It turns the verb phrase 乗る (“to board”) into a noun-like concept 「乗るの」 (“the act of boarding”), so you can say “the act of boarding is scary.”

Once you nominalize with の, you need a particle to show its function. が marks 乗るの as the subject of the adjective 怖い (“boarding is scary”). Using は here would shift it to a topic, and を doesn’t work because you’re not treating it as a direct object.

• で marks the location of an action: 空港で = “at the airport.”
• に with 乗る indicates the vehicle or thing you get on: 飛行機に乗る = “get on a plane.”

Yes, but it changes the nuance.

• 空港で乗る focuses on where you board.
• 空港から乗る emphasizes departure point (“board from the airport”).

少し is an adverb modifying 怖い, meaning “a little.”
You can substitute ちょっと (“a bit”) in casual speech, though 少し sounds slightly more formal or written.

かもしれません attaches to the plain form of verbs or adjectives to express possibility: “might” or “maybe.”

• かも is a colloquial short form of かもしれない,
• しれません is the polite form of しれない.

Yes.

• かもしれない is the plain/casual version.
• かもしれません is the polite version, suitable for formal contexts.

怖い ends in -い and inflects like other i-adjectives (e.g. 怖くない, 怖かった). Na-adjectives, by contrast, require な before nouns (e.g. 静かな空港).