Saul was a false prophet / false apostle
Not sure why you would apply the fallen former King Saul to that category.
I would agree with the great gulf between communing with the dead for the living (necromancy) and the word of God prophecy .All things written in the law and prophets.(sola scriptura) No man can serve two good teaching masters as our one unseen Lord
Are both Abraham and Samuel considered prophets as apostles sent by God ?
Why would you believe Abraham was sent as a prophet? To whom did God send Abraham to in being a prophet? God testified of Abaham in scripture to the nation of Israel as prophesying about them as God's chosen people, which in turn led to the prophesy regarding the seed, Christ, by whom all nations will be blessed and that is how and why all who believe in Him are born of God per John 1:12-13.
So God gave a prophesy regarding Abraham, but I do not understand how you would categorized Abraham as a prophet other than to his own blood line like he did to Isaac in testifying that God will provide for His servants.
Genesis 22:7 And Isaac spake unto Abraham his father, and said, My father: and he said, Here am I, my son. And he said, Behold the fire and the wood: but where is the lamb for a burnt offering? 8 And Abraham said, My son, God will provide himself a lamb for a burnt offering: so they went both of them together.
13 And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son. 14 And Abraham called the name of that place Jehovahjireh: as it is said to this day, In the mount of the Lord it shall be seen.
I suppose Abraham could be seen as an apostle in our eyes in regards to his own household, the future nation of Israel.
Samuel would definitely be seen as a prophet sent to Israel but maybe as an apostle only to our eyes in looking over the steadfastness of the nation of Israel not to go astray.
But I fail to see why it would be important to give either one of them that title of apostleship, or for that matter, Abraham as a prophet when generally we think of prophets as sent by God to speak unto the people.