All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If you carefully read the fatwas that we referred you to, you will find a comprehensive and convincing answer.
You should know that Istighfaar means seeking forgiveness from Allah, and this does not mean that the one who makes Istighfaar fulfills the conditions of Tawbah (repentance).
There are pillars for Tawbah, and when they are met, Tawbah is valid. These pillars are: giving up the sin, being determined not to commit it again, regretting having done it, and giving back the rights of the people if the sin is related to the rights of other human beings.
If these conditions are met, then Tawbah is valid.
Salaat At-Tawbah (Prayer of Repentance) or anything else is not a condition for making repentance; only what we mentioned above is of the conditions.
Nonetheless, Salaat At-Tawbah is a good thing, and it is one of the reasons for attaining forgiveness, Allah willing.
Allah knows best.