Sheikh Muhammad Al-'Ansi ḥafidhahullāh responds:

وعليكم السلام ورحمة الله وبركاته Regarding the first matter, which concerns your father not praying except rarely despite knowing that prayer is compulsory and one of the pillars of Islam, and that when advised, he responds with "Insha'Allah", then it is advisable to take advantage of his acceptance, even though he may generally not be receptive to advice. As long as he says "Insha'Allah" and perhaps requests supplication for guidance, then seize this opportunity to encourage him by mentioning the virtues of prayer, such as its role in expiating sins, elevating ranks, and its fundamental importance in defining a Muslim. Emphasise that abandoning prayer removes a person from being considered a Muslim and highlight the severe consequences of neglecting it, using evidence from Islamic teachings. Additionally, highlight that prayer is a direct connection between the servant and Allah, bringing peace and removing worries, and that it is among the greatest acts of remembering Allah. Mention that all of Allāh's creations, including humans and jinn, are created to worship Him, and that even Shaytan was expelled from Paradise for refusing to bow to Adam out of pride. Pray to Allah to guide him. If necessary, seek the help of some righteous men to advise and encourage him towards goodness. Allah is the Helper. Regarding the second matter, if your has not benefited from the advice and continues to neglect prayer, then according to some scholars, it is invalid for him to act as your guardian in marriage. A person who does not pray is considered a disbeliever, rendering his guardianship and any marriage contract he performs invalid, as a valid marriage requires a guardian who is Muslim and just. If he persists in abandoning prayer, he cannot serve as a legitimate guardian, and thus, the marriage contract would be void. Allah knows best. @madrasatuna