To the first and second part, yes, we mask near constantly, except in certain online spaces, such as this one or a certain discord server we have with Pluralkit.
As for the third part, that is very much a case by case thing. Some of us mask incredibly easily, our host and co-host most predictably are great at masking. Some of us are abysmal at masking, one alter has a very noticeable American accent and his mannerisms aren't subtle in the slightest, plus he's about 30 years off the body's age. Another one loves fish puns and is very upbeat near constantly, which means he draws attention to us. Both of them are usually barred from fronting in unsafe environments. Then there's the rest of us, who are decidedly average at masking. Take myself, (Brightney) for example. I am older than the body's age, but not by too much. I have "nerdy" hobbies, which align with what is expected of us, and I'm able to mentally adjust my language (with a second or two to process) to better match what people expect, more slang, less odd questions and sayings and the occasional curse word to make sure. It's not full proof, anyone who knows we're a system (which isn't many) would likely catch on, probably not able to tell it's me exactly, (I don't front often enough to establish myself like that outside of the system) but definitely aware that I'm not the host. For the most part though, I, and many others insys, would fool most singlets or unfamiliar systems into thinking we too are singlet.
question 111: can you mask your plurality? do you? are some of you better or worse at it than others?