Why is it by some of the kri and ksivs in the megilla - like Memuchan and kiblu, we just say the Kri and other ones I've seen some actually say both the kri and the ksiv - the Baal Koreh reads be-amram, ki-amram (third chapter, fourth verse) or bifneihem, lifneihem (looking up chapter and verse).? What’s the source for all of these?
Every word in Tanakh has a Kri (the way it is read) and a Ktiv (the way it is written). In the vast, vast majority of cases, those are the equivalent to each other. Sometimes they are different. (Very, very rarely one of those values will actually be null, but never in Esther or the Torah.) In every single case we only read Kri and only write Ktiv.
In your case (Esther 3:4) there is a small debate what the Kri for that word is. Almost everyone thinks the Kri is כאמרם different from the Ktiv. A probably mistaken minority opinion [allegedly] thinks the Kri matches the Ktiv there. Therefore in recent years some people decided to be stringent to read both just in case.
The same sort of story applies to a handful of other places in Esther where some minority opinion exists or allegedly exists or historically existed that a word should be read in some other way than the dominant traditional position dictates (eg. 1:22, 2:9, 7:6, 8:11, 9:2). CYLOR before doing jumping on the bandwagon to see if such a practice is worth your time and worth promulgating doubts about our Mesorah of Tanakh.