Is the fiancée considered disobedient to God if she refuses to kiss her suitor? And is kissing considered forbidden if she agrees? And is their sitting together considered khalwa (unlawful seclusion)?

There is no harm in the husband secluding himself with his wife and kissing her after paying the dowry or part of it. As for before paying the dowry, there is also no harm in it, based on what Aisha, may Allah be pleased with her, narrated. However, jurists mentioned that the wife has the right to refuse entry until she is given what was agreed upon to be expedited, citing the Prophet's (peace be upon him) prevention of Ali from consummating his marriage with Fatimah until he gave her something. If the wife consents to consummation and seclusion before receiving the dowry, then it is permissible.