Thanks for pointing to the imaginative connection between the prayer and the manna stories. You point is not only true, it enriches our appreciation for the prayer. Now, on to the next phrase.
"And forgive us our debts, as we also have forgiven our debtors." (Matthew 6:12).
I think it best to view the petition against the backdrop of the jubilee year (see Leviticus 25:8-17). The jubilee year ideal called for the forgiveness of all debts and the restoration of all property to its orignal owners every fifty years. Insofar as I know, the ideal was never put into full practice, but it certainly gripped the imagination of many. Such a practice would have had the practical effect of placing everyone on a equal footing once per generation. In addition, the practice probably would loosen attachment to and dependence upon property, turning followers into stewards rather than owners. Applied on an empire wide basis, it would have involved the Romans leaving the region (though surely disputes would have arisen over the question who originally owned the property!). In short, the petition carried economic and political implications.
That being said, we ought not neglect the personal dimensions of the prayer. Jesus places responsibility to start the process of forgiveness on us. We pray to be forgiven our debts in accordance with how we have forgiven the debts owed us. At this point, I think the petition extends its reach beyond economics and politics to the realm of personal relationships.
For example, Jesus seems to be saying to his first century Jewish listener that he or she must practice preemptive forgiveness, whether of an offending fellow Jew or even a Roman. He calls for breaking the all too common human cycle of revenge and what we might call "compensated forgiveness" in favor of something more radical. Much to our consternation, Jesus never implies such forgiveness on our part guarantees a like response from others. On the other hand, if we take his way we at least drop the terrible burden of holding mortgages (of all kinds)on others.
Does the petition mean we cannot be forgiven by God until we learn to forgive others consistently? I would not put it so. Rather, it seems to me we cannot experience (recognize, absorb, be changed by) and live knowingly in God's kingdom without practicing forgiveness. I suspect learning to practice forgiveness of all kinds is an integral part of growing from infancy to maturity in the kingdom of God.