1. It depends everyone is surrounded by an aura (forcefield) if you close off your aura you will not be able to exchange energy with anyone. You must have met the type of person who seems very removed and introverted. Their energy is very internalized and they don't exchange energy freely, they're hard to reach or connect with. Although, by an aikido-forcefield I take it you mean an aura of loving protection where you respect and try to understand everyone's viewpoint without prejudgment. Where your "forcefield" is expression and an expansion of your natural goodness, and you seek not to dominate but to be in harmony and understanding with everyone. Then you should be able to exchange energy with your favorite tree, and openly exchange your energy and your heart with other people.
2. An aura or field of ki always surrounds you, so you never have to technically activate your "forcefield". Although, if you have the sensitivity you can tighten your aura so that a "ki-ball" attack can't get through. Better yet you can try to absorb the attack and make it part of you or pass over you harmlessly. It's like when you feel threaten by someone or new ideas (you know what I mean?). You can either be scared and hide in your shell, or you can embrace the idea or situation taking what makes sense to you and letting the rest pass on by.
3. From my studies, no. A tree naturally extends ki and is centered. It has not conscious will so it cannot direct its ki, it just is. A human on the other hand has to relearn how to extend ki naturally and stay centered. With exercises like Shomenuchi Ikkyo Undo we relearn what all other parts of nature do naturally. It might seem unfair, but it is this that allows humans to direct our will/ki into endeavors like science, philosophy and yes making balls of ki.
4. Again, a "forcefield" or aura surrounds everything if you prefer. You can extend your aura over the tree and have it resonate at your "frequency" or you can attempt to blend your energies together. Kind of like when you hear a good speech and you can feel the energy from the speaker wash over you.
5. Everything with a life-force has meridians where ki concentrates in the physical vessel. Where these meridians intersect or accumulate pockets of ki a "pressure point" or node is created. The Chinese used this to develop a method of holistic medicine that predates our medical sciences by a thousand years. Although, most of their research is on the meridian system of the human vessel, I don't think it's a giant leap to believe that trees have a meridian system of their own. It would make for some interesting research. Western medicine is just starting to adopt some of these practices. I can't wait for a time where the best of western and eastern medicine merge together.
6. Yes. Reiki practitioner extent energy into there patients all the time. Certain styles of Chinese martial arts (Snake animal form for one) teaches its students to send negative ki into a pressure point strike (Kind of like a poison). Don't we as aikidoka extend ki when we do Yonkyo?
7. Yes. You can direct ki to parts of your body including pressure points. When you do Ikkyo Nage don't you extend your ki through your body and out of your hands?
Question: If the essence of budo is the spirit of loving protection of all beings of the universe, and to harmonize ourselves with the movement of the universe and bring ourselves into accord with the universe itself, when we make fun of peoples belief (Be it religious, metaphysical, or philosophical) are we practicing good aikido?
Aikido should be practiced outside of the dojo as well.