Can one person's karma be affected by the karma of another person or other persons?
The short answer is “Yes”… Olivia Glad,… because the others provide the grounds, the right conditions for a kamma to bear fruit. How this comes to pass needs some explanation.
The laws of kamma are not just based on causes and effects, they depend on conditions. This is what prevents laws of kamma being deterministic. Understanding this would make it clear that one’s future is not determined by one’s past actions or kamma alone. Thus past kamma do not necessarily lead to kamma vipāka.
What first gets created is a kammā beeja, or a kammā seed. For that kamma beeja to bring about its result (kamma vipāka), suitable conditions must exist.
We can avoid many possible bad kamma vipāka by being mindful and not providing conditions for them to appear. We can also make conditions for good kamma vipāka to bring about good results. If one goes into a bad neighborhood at night, that is providing conditions for any suitable bad kamma seeds waiting to bring about their bad vipāka.
Once a bad vipāka materializes, we can certainly manage it to minimize its effects, and in some cases even get rid of it by working to making conditions for a good kamma seed to overcome it. Kamma will not bring about any kamma vipāka, if and when one attains the Sotapanna / Stream Entrant stage. One can then erase all kamma vipāka that would lead to the four lower world realms.
One person's karma can have effects to another person. It is like living in/near war zone where a lot of killing (people are doing bad karma) are happening. The chances are very high to get killed/injuries. On the other hand, if someone don't have enough bad karma to get killed, he/she will escape no matter what. It just needs the right condition to be affected by (present and past) karma .
No, not really (either in regard of kamma, action, nor in regard of vipaka, results of ones action, as they are sure to ripe according to ones actions). One could just provide with offering certain circumstances, that ones past or present kamma could ripe or not. Of which one has certain free choice to do or not as well. While some one could give, offer something, it's at least up to the other individual if taking, able to take, see..., or not.
"Beings are heirs of their actions..." disciple of the Buddha would recitate every day, following his teachings.
Doing what ever, if you like to be angry or stubborn, it is just you how is able to change that. No one else could make you happy. Nobody can make decision for someone else.
While suffering or happiness is ultimatly a matter of touch (of either knowing or not-knowing) it's nevertheless importand to develop gratitude toward giver of liberating things, since inclination and nurishing association with good people would increase the chance to be offered the path to the highest liberation, a "a luck" seldom to meet, a Sublime Buddha and rest of his heritage, carried and offered by his good disciples.
Whether this answer might touch you physical, might touch you at proper time, when proper attention is up, that lies within your sphere and is ground to rejoice with your merits if having been able to take, wishing to get touched by such again and again.
And if looking for certain offerings by answeres, it's because of Upanissaya that one tends to associate which the wrong again and again. No one could change their inclination to act.
[Note: this is not given for trade, exchange, stackes, Buddh-ism or other wordly gains, but simply a matter of compassion for those having Nissaya]