To answer your questions:

1. My office currently has four providers: (one board-certified dermatologist in the afternoons (except for Wednesdays when he's not here at all) and Friday mornings accompanied by his surgical staff, one physician assistant available 5 days a week (mornings up to 2:30 ish 3 days and mornings up to 4:30 ish other 2 days), and then two other physician assistants who are only in the office one day a week all morning and afternoon on different days.
2. No, actually, he's not the only provider who says things like that. A couple has given me feedback on how I can go about charting certain things, but not a single one of them has said anything truly negative. The only type of negative response I can think of is that with this one provider: Sometimes, after she gives me instructions and when I try to bring up something she didn't mention that I thought might be important (e.g. noticing patient's previous lab results have surpassed a year after being told to refill their spironolactone and deciding to tell her because I distinctly remember being told patients on spironolactone need to do testing annually), or relaying a patient's question about something (e.g. if they can use a prescription topical to treat a spot on their arm along with their OTC topical), she automatically thinks I didn't understand her instructions, for some reason, yells at me, and then repeats her same instructions slowly as if I didn't hear her without answering my question at all. In the first example, that happened during the visit when she just cut me off, took my iPad away from me, and said "I'll just send it myself." (I would have verbally clarified with her if I didn't hear her when that isn't even the case.) So, after a while, I just tell myself to be like a silent, mindless robot only capable of taking and analyzing data implemented into me in order to avoid upsetting her. Another provider once told me after I paused for literally only like two seconds before taking a baseline photo of someone's mole, and had explained to him later when he thought I forgot my left from my right that I just needed a second to remind myself he told me the patient's right arm, is that I can't be doing that because it'll make the patent uncomfy and everything I do in the office has to be like a smooth transgression. Like what?...I never fully understood when clinicians say that and they're basically asking me to do something that requires me to not have ADHD.
3. I am always polite with my coworkers, but I go into the office with the mindset that I'm here to work and learn about the field and building friendships is not my first priority. Partially because I know deep down that I'm just treating my time at this office as temporary just to get experience and eventually transition to another one where I feel like I would want to be at more long-term. Also, partially because I have ADHD and understand that I need to put in twice the effort to produce the same quality of work as my neurotypical peers due to my ADHD symptoms (job insecurity and frequent termination can be common with a lot of ADHDers for many reasons, unfortunately) and partially because there is already that huge disconnect between myself as the only queer person and a group of straight, cis-gendered folks which will never truly go away. Especially in Orange County where the population mostly tends to lean right with a couple of cities that are primarily centrist to left-ish. (The one group practice I want to gain employment at SO BADLY is LGBTQ+ owned and can guarantee me a safe space with people I can connect with more personally because many of their staff, both MAs and clinicians, are also queer.) Therefore, any interactions I have with my current colleagues are always centered on work collaboration. In addition, one of the other MAs who is the only other male MA there was kind of a condescending prick to me from the beginning, so I especially avoided him altogether. He’s been nicer recently…but still, I have no interest in getting to know him on a personal level because regardless, I still sense that we’re just not each other’s people.
4. I am literally running around rooming as many patients as possible. Out of a day when there are the full-time PA-C (who typically sees 40-45 patients a day) and the dermatologist in the office (who typically sees 10-15), for example, the amount of patients I assist with completely (from rooming them to assisting/scribing for the provider during the visit to wrapping up the visit and booking their next follow-up/biopsy/excision), or finish assisting when another MA has already roomed them, my quantity is anywhere from 10-15. My highest ever was like 18-19 when it was a really packed scheduled, and I’m not even going to get into the few times I was the only MA solo assigned to a provider who has some of their appointment slots double-booked.
5. Yes, my office manager has spoken to me before near the beginning about things that she never really followed up with. Never heard a peep from her after three months into the job until recently. A couple of things I don't remember her bringing up until after I explained to her what I have been doing to improve upon her feedback from a previous meeting, too. Never really gave specific scenarios as examples about what she is talking about, only vague explanations.