1.After a 5 years of relationship with a girl can I marry her to protect her name and avoid to spoil her name in the society going against istikhara if it has come no ?
2. Is there any other solution if i get istikhara negative and still want to marry her ?

Marrying anyone is not issue, for marriage first need research then go for estekhra, and when u rely on estekhra then why engaged someone 5 yrs?
If your family satisfied and her family, then give sadqa and get married.