1) He can depend on best estimation.
2) No. He can not give it as sadaqa on their behalf, except if he does not know who they are, or if he is not able to reach them or their inheritors, by any means.
3) He can pay gradually.
4) For such huge non-Muslim corporations, no need except tawba
5) he can make best estimation, and pay it as sadaqa on their behalf.
6) As explained before, he can return the money as gift. It is not necessary to tell what happened.
7) If they forgive him, then he do not need to worry.
8) He can tell him that he paid the amount as sadaqa on his behalf. If he accepted, then fair enough.
9) He have to be specific, as this is material issue not immaterial like talking about him in his absence.