But verily, he was a sign of the Hour. So have no doubt about it, but follow me. This is the right path.(Chapter 4 Verse 63)
The second part of the verse says so have no doubt about it referring to the hour and it is obvious he is addressing the disbelievers that after witnessing this specific sign how can they possibly have any doubts in regards to the hour? This verse is a sign for disbelievers. If your argument had any basis then on day of judgement everyone who saw the Quran and rejected it can simply say no we did not witness the sign of the hour so we have doubts and do not believe. If the hour has not come how can they believe it? and be expected by Allah not to doubt it?
Secondly if innahu referred to him coming Allah would not say So have no doubt but follow me right after the statement. If it was for Isa(as) Allah would have said but follow him or obey him but no he referred to himself .This sign of hour refers to the Quran! the greatest miracle. It spiritually gives life to the dead and Allah tells us this In chapter 25 verse 50:
That We may thereby give life to a dead land, and give it for drink to Our creation — cattle and men in great numbers.(Chapter 25 Verse 50)
In Tafsir Mu’alimut Tanzil, it is stated:
قال الحسن و جماعة انہ یعنی ان القرآن لعلم للساعة
“Hadrat Imam Hasan (ra) and a community of people state that
Innahu infers that the Holy Qur’an is the Sign of the Hour.”
Further in Tafsir Jami’ul Bayan it states”
و قیل الضمیر للقرآن
“Some have said that the pronoun in this verse refers to the Holy Qur’an.”
In Tafisr Majma’ul Bayan it states:
و قیل ان معناہ ان القران لدلیل للساعة لانہ آخر الكتاب
“Some have said that the meaning of this verse is that the Holy
Qur’an is a proof of the imminent resurrection, because it is the last book.”
Holy prophet(saw) is also a sign of the hour and this can also possibly refer to him. Allah tells us in Chapter 8 verse 25:
O ye who believe! respond to Allah, and the Messenger when he calls you that he may give you life, and know that Allah comes in between a man and his heart, and that He it is unto Whom you shall be gathered(Chapter 8 Verse 25)
Innahu can indeed refer to Jesus(as) as well as Mirza Ghulam Mirza Ghulam Ahmad(as) has stated in Hamamatul Bushra and Ijaz-e-Ahmadi. Mirza Ghulam Ahmad(as) explained that Jesus(as) was a sign of the hour for the people who rejected the day of resurrection.The Sadducees did not believe in resurrection and were a Jewish sect which is proven by bible mark 12:28 explains their reply to jesus. Sa’ah is also used for punishment so Jesus was a sign for children of Israel due to the disobedience and persecution towards Jesus
Here is the quote from Mirza Ghulam Mirza Ghulam Ahmad(as) from Hammamatul Bushra:
“Let it be known that in relation to the Messiah (as), Allah the Almighty states
Innahu la‘ilmul-lis-sa‘ati [i.e., Verily he is a sign of the Hour] and not Innahu sayakunu
‘ilman lis-sa‘ati [i.e., Verily he shall be a sign of the Hour]. Hence, this verse indicates that he was a sign of the hour for the very reason that he practically possessed that sign within him, and not because that sign would be proven true in his person later on in time; and the reason he was a sign was because he was born without a father. The detail is that a Jewish sect known as the Sadducees disbelieved in the resurrection. As such, Allah informed them through the words of various Prophets that a boy would be born from among their people without a father; and he would be a Sign for them to establish the truth of the day of resurrection. It is to this very fact that the verse ‘Verily, he is a sign of the Hour’ refers to. Similarly the verse ‘And we made him a sign,’alludes to the fact that we would make him[i.e., Jesus] a sign for the Sadducee people.”
Even if one was to use the name of Jesus(as) with Ilmun in its infinitive form as its being used in this ayah, it would only mean that Jesus(as) was a prophet of God who had knowledge of the day of resurrection, nothing more.