1) This caused by the v5 plugin that will ask you to send a mail after saving your article, but you have unpublished the Acymailing v5 component. Please note that this function is not (yet) available in v6.
2) You're correct, you can't show the intro text. Hoever, it is possible to specify the number of characters you want to show as intro text.
3) There is no css file you can edit within Acymailing. For now you have to work with inline css or, as you already found, add a custom style sheet. Based onm your needs I would suggest to use the custom style sheet. Please not that it is recommended to add !important to all your css in order to prevent it gets overridden.
I know Acyba will add a possibility to set these kind of css as default parameters in one of the upcoming v6 versions. Please be patient.