Also known asweddingvivahshaadi
Vastu poojan is done not only at the start of construction but at several stages and family events; at griha arambh and griha pravesh it is essential.
The whole house, with an offering at every devta pad
No. Do it at bhoomi poojan or shilanyas, when the first slab is laid, when the main door frame is installed and on griha pravesh; the start of construction and griha pravesh are essential. Also do it after a renovation in which walls are broken, for a marriage, the start of a job or business and the birth of a child, and once a year on a swayam siddh muhurta such as Akha Teej or Gudi Padwa for a year of happiness and peace.
Yes. After marriage, as in the hathleva ritual, the planets of one partner overlap the other, and the exalted planet's influence prevails.
The janma rashi runs the whole life, so all stable works such as the muhurta for building a house and marriage are done by it. If the name is not from the janma rashi, its rashi gives a different element, which can be used in business, because bank, GST and tax records carry that name.
Manglik has nothing to do with it. It is considered only at marriage matching, when Mars sits in the 1st, 4th, 7th or 12th house, and Mars is power, not inauspicious. For the SW check whether Rahu-Ketu are debilitated, and for the South whether Mars is bad.
No. What counts is composition and quality, not appearance; ashopalav leaves have a different composition, so they do not carry the properties of mango leaves. Mango leaves have many special qualities: their juice is used for purifying land, and where marriage is not happening or auspicious functions keep failing, mango leaf juice is applied as a lepan beneath the place where the girl or boy sleeps, after which the auspicious events take place.