Ya know, I don't know for certain. I am new to this and still learning. Some people think that some spirit guides are relatives or people you knew in life. From my reading, spirit guides are human, but have ascended to a higher spiritual plane. Some lived many lives. I have also read that it is possible that you knew your birth guide in a former life.
It is my understanding that your birth spirit guide and you chose to be together before you were born, others come to you as needed. He/she will be with you always. Mine said he chose me to help protect me from my Dad. My guide died 3500 years ago and lived in Sameria(re: the good Samaritans of biblical fame). I have two that help me with communicating and spirits. And I have one who is either a guide or my higher self - I'm not sure about her. I have not met any of the others.
So far, all of them let me know they are with me by the tingles. I suppose it's the crown chakra that actually tingles. The whole top of my head tingles when my main guide is here, the right top with the others. When I am in contact with spirits, the left side of my head and face tingle, depending on who's here. It has been a while since I've talked to any of them (too chaotic).
I ask complete questions expecting complete answers - not all yes or no. The most I get are 1 -3 word answers. I started with asking for a guide to come to me - it was my birth guide. I asked him to make somewhere on my body tingle if he was truly with me ( I did this a couple of times so I knew for sure). I asked his name, whether he was male or female, where he lived, where he died, stuff like that.
As to your other questions - I don't know. I have not asked any future questions. I don't know if they can do actual physical healing. They are there to guide you and help you with life choices. They suggest and guide us to the best way, but because of our free will, we are responsible for creating everything in our lives. We can choose to follow our gut feelings(their suggestions) or go in a different direction - it is up to us to create the best life possible. Because I have been proactive in correcting a future health problem, my birth guide praised me - I was doing a good job and to keep it up. In that sense, that much of our future is made known to us. Don't know.
As far as altering the future, again, I don't know, but I have an incidence that tells me my guide is with me always. He literally kept my car from colliding with a stupid, thoughtless kid's car by mere inches. The kid pulled into my lane too close for me to have stopped in time - we were coming to a red light at an intersection.
The spirit guide group has some excellent articles to link on and learn from. Hopefully other more knowledgeable members will help you. I can only give you what I have learned.