Unfortunately, the answer to many of your questions above is "it depends". From my own experience and observation, working in a lab in the field of your interest certainly is more ideal, but across research fields, the principles, daily routines and administrative hurdles are more or less the same. You will get a sense of the life as a researcher in general no matter which lab you land in, the difference is probably just in the flavors.
As a volunteer, you'll likely start with basic housekeeping work, glassware, gels, PCRs, etc., repetitive work with little intellectual contribution. It sounds frustrating, but keep in mind that with your lack of research experience, you bring only a pair of extra hands to the lab, and lab members have to carve out time from their own stressful schedules to teach you how to pipet. It's a slow and boring start, and many people have stopped and turned around there. Should you be able to catch up on scientific knowledge in that field and master whatever basic techniques commonly employed in that lab, you may convince a lab member to let you help with their project. Under their supervision, you should be able to plan, design, and execute experiments aimed at answering distinctive questions that contribute to the project. If you excel in this stage, you may already have enough experience and skills to land a paid research assistant position, either in the same lab or another one. How long would it take you to reach this point is mostly up to you. The faster you assimilate knowledge, the better you are at scientific critical thinking, the more rigorous you are at executing experiments, the shorter it takes you to reach the end of the tunnel (and the beginning of another infinitely longer one called the research career).
Grades may not matter as much as GRE and research letters. I'll leave it to others who have experience applying to PhD-only programs.
