Peace be on you.
1=All Prophets including Ibrahim, Moses, Jesus, Muhammad , peace be on them] were human and pure of sins, they constantly sought cover and help from God. The word sin for them is not like the word sin for ordinary people. They took care of minutest deed should not be in error yet they were human, they always sought cover and help of God.
" Istighfar means to cover up, protect or suppress. The primary connotation of Istighfar is to supplicate for the suppression of human tendency to fall into error so as to be safeguarded against error or sin. Its secondary connotation is to supplicate for suppression of consequences of error. Thus Istighfar might connote suppression of a person's tendency towards falling into sin or error, or his protection against the consequences of such errors or sin, or of the errors and sins of others.
On account of lack of knowledge of Arabic, the true significance of Istighfar has been lost to many. This word is derived from Ghafr, the other infinitive forms being Maghfirat, Ghufran etc. As given in all Arabic dictionaries, the word ghafr means - to cover or to protect. Therefore the primary meaning of Istighfar mean, seeking protection or a prayer of protection and support. All other meanings usually given to this word are derived or secondary."
Ref:
https://www.alislam.org/library/links/00000111.html
2=
Just for the sake of argument [though we believe Jesus was true Prophet]
If you see from the view of the people to whom he came:
"And if a man have committed a sin worthy of death, and he be put to death, and thou hang him on a tree" is "the accursed of God". (Deut.21:22-23)
The Christians also believe that Jesus was hanged on the cross and died an accursed death. Says Paul:
"Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, cursed is every one that hangeth on a tree". (Galatians 3:13)
3=But the Holy Quran declares that Christ did not die on the Cross. God saved him from death by crucifixion in the same way as He saved His dear ones from tribulations, Did not Jonah the Prophet come out alive from the belly of the whale?
The fact of the matter is that Jesus Christ only fell into a swoon on the Cross, but when he was taken down from it, he recovered then died a natural death at the good old age of 120.
Ahmadiyya-Muslim understanding @
https://www.alislam.org/library/dcross.html