Regarding your first question of whether you are allowed to come back to bentch where you ate, the answer is that you certainly may come back and bentch. The more complex question is whether you are required to do so.
Beit Shammai[2] rules that if one forgot to bentch at the end of his meal and remembers to do so only when he is already in a different location, he is required to return to bentch in the place where he had eaten. Beit Hillel, on the other hand, maintains that one is permitted to bentch wherever he remembers. If one purposely left his place of eating without bentching, the gemara[3] says that even Beit Hillel agrees that he must return to the location of his meal.
The Shulchan Aruch[4] cites two opinions from the Rishonim as to whether we accept the ruling of Beit Shammai or Beit Hillel regarding returning after forgetting to bentch, and the Shulchan Aruch does not present a clear indication as to which opinion he accepts. The Mishna Berura[5] comments that even the lenient opinion agrees that it is preferable to return to one’s original place, and the Mishna Berura thus concludes[6] that one should indeed return if it is not difficult to do so.
If one eats even a small amount[7] of bread in his new location, then he need not return to bentch in his original place, because the new location becomes his place of eating.[8] Indeed, he preferably should bentch in the last place he ate.[9]
Regarding until when one may still bentch, there is no difference whether he is in his original place or in a different one. The mishna[10] says that one may bentch until the “food has been digested in his intestines.” The accepted explanation is that this means until a feeling of hunger has begun to return.[11] This time limit is obviously a function of how much one ate; in any event, it is difficult to pinpoint the exact moment at which one again “feels hungry.” Therefore, we usually work with the assumption that one has, from his last eating, the amount of time it takes to walk a distance of four millin, which most authorities hold is seventy-two minutes.[12]
There are other significant questions regarding a person who leaves his place of eating. One such question is what he should do when he wishes to continue eating upon his return. The Shulchan Aruch[13] rules that unless one left behind others who continue eating in the original place, there is a break between the returner’s two eating sessions, and he therefore must first bentch for the first part and then recite a new beracha for the second installment. The Rama[14] disagrees partially. He rules that if the food that one had eaten is of the type that mandates reciting a beracha acharona before leaving one’s location, then his meal is considered to continue, and additional berachot are not required. It is a matter of debate as to what exactly is included in this category of food,[15] but all agree that a bread meal certainly is included. Nevertheless, it is improper to leave the place of one’s meal for any extended period without first bentching, unless there is a pressing reason to do so, such as the need to perform a mitzva elsewhere.[16]
Although Sephardim generally accept the Shulchan Aruch’s rulings over those of the Rama, this is an example of the counter-rule maintained by many Sephardi authorities – that one should not recite a questionable beracha even when the Shulchan Aruch says to recite it.[17] Thus, for a Sephardi, it is especially important not to leave the location of one’s meal with intention to return and continue eating, as this creates a situation in which he will not be able to comply with the Shulchan Aruch’s ruling to make new berachot.[18]
If one had eaten with other people who remain in the original location, his connection to the original eating is certainly a strong one. Thus, the Shulchan Aruch[19] agrees that in such a situation he does not require new berachot. Certainly, if before he left he had an obligation to join his counterparts for a zimun, he may do so upon his return.[20]