1. Regarding chocolate, the leniency depneds upon one's opinion in regard to powderered milk; please see: https://oukosher.org/blog/consumer-kosher/chocolate-covered-in-halacha-kashrus/
"Rav Tzvi Pesach Frank, based upon a psak of the Chasam Sofer (see shut Yoreh Deah 107), was of the opinion that milk in a powdered form was not included in the gezeira of cholov akum (see Har Tzvi Yoreh Deah 103 and 104). However, the Chazon Ish was of a dissenting opinion and assumed that dried milk remains prohibited (see Chazon Ish Yoreh Deah 41:4). There are varying practices today amongst kashrus agencies who otherwise would require cholov yisroel, whether powdered milk falls under the rubric of the gezeira of cholov akum."
2. We are unaware of any leniency due to solely to the difficulty in kashering the machinery.
3. Regarding the leniency on butter, please see the OU's assessment on the leniency for today's times [https://oukosher.org/blog/consumer-kosher/everything-is-in-butter/]:
"Although it is a machlokes among the Geonim and Rishonim, the Rambam (Hil. Ma’achalos Asuros 3:16), Tur (Yoreh Deah115), Shulchan Oruch (YD 115:3) and many later poskim permit butter made from non-supervised (gentile-farmed) milk so long as there is no actual liquid milk residue present. Their rationale is that non-kosher milk cannot properly solidify into butter, and Chazal did not prohibit non-Jewish butter as they did with non-Jewish cheese. Based on this p’sak, many consumers who are otherwise meticulous to purchase only cholov Yisroel products will buy cholov stam (regular, non-cholov Yisroel) butter.
Were today’s butter only made from sweet cream, there would be nothing to discuss, as the heter to use cholov stambutter would clearly apply. However, some kosher professionals have pointed out that since today’s butter often contains whey cream and various other cholov stam additives which – they assert – do not or may not qualify for an exemption from cholov Yisroel requirements, consumers who do not otherwise purchase cholov stam products should not purchase cholov stam butter either."