And whosoever seeks a religion other than Islām it will never be accepted of Him and in the Hereafter he will be of the losers.

Instead of directly answering your points I will instead insha`Allāh make a number of general points here in which the points you raise will be answered by the permission of Allāh.

1. The Disbelievers are in Hell Forever

“Those who disbelieve and die as disbelievers, on them is the Curse of Allāh, the curse of the Angels and all of mankind. They will abide therein and their penalty will never be lightened nor will they be given respite.”
[2:161-162]

“Thus Allāh will show them their deeds as regrets and they will never leave the Fire.”
[2:167]

“Every time they seek to get away therefrom, from anguish they will be driven back therein and it will be said: ‘taste the torment of burning.”
[22:22]

2.The Christians are Disbelievers

“Indeed they disbelieved who say that Allāh is Christ.”
[5:17]

“Indeed they disbelieved who say that Allāh is Christ the son of Mary. But the Messiah, son of Mary said, ‘O Children of Israel! Worship Allāh, my Lord and your Lord.’ Indeed whosoever sets up partners with Allāh then Allāh has forbidden Paradise for him and the Fire shall be his abode. And for the wrong-doers there are no helpers. Surely they disbelieve who said that Allāh is the third of three but there is no god save one god.”
[5:72-73]

“The Christians say: Messiah is the son of Allāh. They imitate the saying of the disbelievers of old, Allāhs curse be on them how they are deluded away from the truth.”

Yes, whosoever believes in Allāh and the Last Day is a Muslim, but can the Christians be said to believe in Allāh? No! Instead they insult Him in the worst of ways and in fact worship something that is not Allāh but a mere figment of their imagination.

As for Christian X being worse or better than Abū Jahl then this is for Allāh and not for us to decide, it is for us to judge on what is outward and as for what is inside a person then that is for Allāh to judge.

Their Actions are Null and Void

“Whosoever rejects faith then his deeds are fruitless and in the Hereafter he will be from the losers.”
[5:5]

“As for those who disbelieve then their deeds are like a mirage in the desert, The thirsty one thinks it to be water until he comes up to it and finds nothing but he finds Allāh with him Who will pay him his due (in Hell).”
[24:39]

4. The necessity of testifying to the Messengership of Muḥammad (ṣallallāhu ʿalayhi wa-sallam) to become a Muslim

“When the hypocrites come to you (O Muḥammad) they say: We bear witness that you are the Messenger of Allāh. Allāh knows that you are indeed His Messenger and Allāh bears witness that the Hypocrites are indeed liars.”
[63:1]

Who is and who is not a hypocrite is known to Allāh Alone. We are just ordered to judge on what is outward, therefore whosoever says the shahādah then he is a Muslim as far as we [Ahlus Sunnah] are concerned and by virtue of his saying the shahādah he is better than the best of the disbelievers.

“Say (O Muḥammad): If you truly love Allāh then follow me. Allāh will forgive you your sins and Allāh is Oft Forgiving and Most Merciful.”
[3:31]

The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam) said to Muʿādh, “your are going to a community of the People of the Book so first call them to testify that there is none worthy of worship save Allāh and that I am the Messenger of Allāh…”[Ṣaḥīḥ Muslim no. 27]

He (ṣallallāhu ʿalayhi wa-sallam) also said, “I have been commanded to fight the people until they testify that none has the right to be worshipped save Allāh and that I am the Messenger of Allāh….” [Ṣaḥīḥ Muslim no. 31]

The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam) said, “I bear witness that none has the right to be worshipped save Allāh and that I am the Messenger of Allāh. The man who meets his Lord without harbouring doubt about any of these two would never be kept away from Paradise.” [Ṣaḥīḥ Muslim no. 42]

The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam)) said, “He who testifies that there is none worthy of worship save Allāh and that Muḥammad is His servant and Messenger and that Christ is His servant and the son of His slave-girl (Mary) and he is a word which He communicated to Mary and His spirit and that Paradise is a fact and that Hell is a fact. Allāh would make him enter Paradise through any of its eight doors that he wishes.” [Ṣaḥīḥ Muslim no. 43]

Whosoever does not testify to the Messengership of Muḥammad (ṣallallāhu ʿalayhi wa-sallam) has not believed in Allāh correctly and in fact he has rejected all the Prophets for to reject one it to reject all.

5. It is Obligatory Upon the One Who Hears of Islām to Accept it:

The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam) said, “by Him in Whose hand is the life of Muḥammad whosoever from the Jews and Christians hears about me and does not believe in me and what I have come with shall be one of the denizens of the Fire.” [Ṣaḥīḥ Muslim no. 284]

6. The Jewish and Christian Religions are Abrogated and Invalid

“Fight against those who believe not in Allāh nor the Last Day nor forbid that which Allāh and His Messenger have forbidden and those who do not acknowledge the Religion of Truth from amongst the People of the Book until they pay jizya with willing submission and feel subdued.”
[9:29]

“Indeed the only religion (acceptable) with Allāh is Islām.”
[3:19]

“And whosoever seeks a religion other than Islām it will never be accepted of Him and in the Hereafter he will be of the losers.”
[3:85]

“This day have I perfected your religion for you, completed My Favour upon you and have chosen for you Islām as your religion.” [5:3]

“And We have sent to you the Book confirming the Scripture that came before it and a witness over it.”
[5:48]

And Allāh the Most High knows best.