Kesin geliyorum.

In this sentence, kesin is used adverbially and means “definitely/for sure.” Although kesin is primarily an adjective (“certain/definite” as in kesin karar “definite decision”), it commonly functions like an adverb in speech.
Kesinlikle is a true adverb and sounds a bit more formal/emphatic.

• Colloquial: Kesin geliyorum.
• More formal/neutral: Kesinlikle geliyorum.

Both are possible, depending on context. The Turkish present continuous (-yor) often covers near-future, planned actions, similar to English “I’m coming/I’m going.”

• Right now/on my way: If context makes it clear (e.g., you’re already en route), geliyorum = “I’m coming (now).”
• Planned/decided future: With a future time or in a planning context, geliyorum ≈ “I’m going to come.” Adding a time word removes ambiguity: Yarın kesin geliyorum.

Both work, but there’s a nuance:

• Kesin geliyorum suits near-future or an arranged plan (often in conversation).
• Kesin geleceğim (future tense) is a straightforward promise/commitment about the future and fits well in neutral or formal contexts, especially in writing or when the time is clearly future.
  With a clear future time, both are acceptable: Yarın kesin geliyorum / Yarın kesin geleceğim.

Yes, but the aorist -r (gelirim) carries a slightly different feel: it can sound like a general promise/willingness or a confident prediction rather than a set plan.

• Kesin gelirim ≈ “I’ll certainly come (you can count on me).”
• Kesin geleceğim = a direct commitment to a specific future instance.
• Kesin geliyorum = an arranged/decided plan, often near-term.

Yes. Turkish usually drops subject pronouns, but adding ben emphasizes contrast or focus on “I”:

• Ben kesin geliyorum = “I, for my part, am definitely coming (even if others aren’t).”
  Without ben, it’s a neutral statement.

Default placement is before the verb phrase: Kesin geliyorum.
You can also put time and other adverbs before the verb:

• Yarın kesin geliyorum.
• Kesin yarın geliyorum. (also fine, but many prefer time first)
  Post-verbal Geliyorum kesin is unusual; it might work as an afterthought with a pause/comma: Geliyorum, kesin. In standard usage, keep kesin before the verb.

Use the negative of the same tenses:

• Present continuous (often for this event/plan): Kesin gelmiyorum.
• Future (clear future commitment): Kesin gelmeyeceğim.
  Both are strong; Kesinlikle gelmeyeceğim is an even more forceful formal option.

Yes—kesin signals certainty. If you’re less than 100% sure, soften it:

• Büyük ihtimalle gelirim. (I’ll most likely come.)
• Galiba gelirim. (I’ll probably come.)
• Sanırım geleceğim. (I think I’ll come.)

Use either present continuous or future, depending on nuance:

• Kesin geliyor musun? (Are you definitely coming [as planned/near future]?)
• Kesin gelecek misin? (Will you definitely come [future commitment]?)
  Both are natural.