shaatnez,tatoos,kosher,marit ayin and more

Question

may one wear a wool sweater on a linen shirt or vice verse ? do married women need to cover their heads [in private] for saying a bracha or prayer [like men] ? when do we say something mutar is a problem of marit ayin and when not..it seems so subjective ? may ne get a 'short term' tatoo.....it last for a few weeks and then fades...its different thana regular tatoo in that it is injected on an upper level of dermis where the dermis can regenerate..is this a tatoo too ? can a non religious jew who is behind a counter [or in general] be believed if he says something is kosher ? a non jew ? [what if either has no monetary interest in itt [ie a waiter or a non jewish/non religious friend] thank you!