yes
No, the word "sincerely" is not capitalized unless it is used at the beginning of a sentence or in a heading.
sincerly do your work
i sincerely apologize for my grievous error.
She gave her acceptance speech with sincerity.
i sincerely believe in you.
We didn't work out as a couple, but I sincerely hope she finds the right person someday. I regret the accident and sincerely regret any pain I may have caused.
can you start a sentence using the word phishing?
Yes, you would do that cause you are talking about and to somebody else.
When Shelly hurt Clair she was sincerely sorry.
Rosie sincerely regretted misleading her online boyfriend, but she felt that it was finally time to tell him the truth--she was actually just a groundhog who happened to have WiFi.
No the word sincerely is an adverb.
The superlative degree of word sincere can be most sincere ,or sincerest according to its use in the sentence.