Yes. This and these can be determiners or pronouns. They can be subject or object pronouns.
Hey look at this!
What do you think of this?
there is a famous nava message"Sighted ship, sank same"
okay
Yes you can. Example: Okay, I will go with you.
Yes.
I am photofinishing this okay
Yes.
Yes. There is no English word that cannot end a sentence.
Okay, this is my drabbest shirt.
noA sentence cannot end with the word "the". Hmmm, wait a minute.
The word "incidentally" can be used at the end of a sentence. You can make the sentence "This was done incidentally.".
okay here is a sentence: "I don't know what prognostication means!"
Ending a sentence with the word "at" is generally not considered correct grammar. It is better to rephrase the sentence to avoid ending it with a preposition like "at."