a) there are some ainu words which crossed over to japanese, but i(and presumably linguists too) have no idea how many, and as far as i can see most of these words are just fishe names
b)ainu names are radically different from japanese ones, but i dont know much about them. if you see an ainu name in katakana, then thats their real name, if you see an ainu name in kanji, then thats either "ateji" or a japanese name that they selected themselves
c)you should be able to tell that somebody's an ainu the moment you learn their name, if they use their ainu name or have ateji'd it then you'd notice immediately(though you might confuse it with some other foreign culture's name or something), if they picked out a proper japanese name and surname for themselves then theres no way you could notice. though, there are some ainu who pick out a japanese name or ateji their ainu name, and have their surname remain as is
i assume most japanese never encounter an ainu in their life since the ainu population in hokkaidou according to a 2017 survey was 13k
some interesting things i saw while searching was that around 80% of the place names in hokkaidou come from ainu(ateji'd or directly translated), and some of the place names in touhoku, karafuto(sakhalin), kamchatka, and the islands between karafuto and kamchatka come from ainu too, though there's not many in the russian side because they're evil kikes who rename traditional ainu land
"ateji" is randomly giving kanji to a name that is not japanese, according to how you would pronounce it in japanese. for example(while most people just write 米 or アメリカ(america in katakana)), america in ateji is 亜米利加, 亜 is pronounced a, and basically has the meaning of the "sub" in subhuman, subspace etc
米 is normally pronounced mei, though here it's "me" instead, and means rice
利 is pronounced ri, and means benefit
加 is pronounced ka, and means addition
the vast majority of the time ateji is given without caring about what the end result will be, for example the "sub-rice beneficial addition" is utterly retarded nonsense
most country names are given ateji like this, and then the first kanji of their ateji name is used to represent them. in the case of america though instead of using the 亜, they use the 米, because if they used the former then saying "american" would be the same as saying "subhuman". when you say canadian etc, you just add 人 after the first kanji, but if you try doing that in the case of americans you end up with 亜人, which quite literally means subhuman(note that there are some fascists commies etc who call americans 亜人 intentionally)
ateji occurs in peoples names too, though in this case you dont just give people ateji against their will, they choose if they want to have one, and if they want to then they pick out the kanji themselves. 阿弖流為(aterui) is a famous emishi(pretty much just south ainu folk) who integrated with japanese culture and got himself an ateji name and a post, then later rebelled. the kanji are distributed a-te-ru-i, and this one doesnt make much sense either