High philosophy is too much for me.
Here the question is simple. A Hindu boy married a Christian girl. The boy’s father objected first. Now he has reconciled. He wants the girl to become Hindu. The girl is willing. Then where is the problem? The boy and girl were husband and wife. If the girl converts, still they are husband and wife. The boy’s father wanted the girl to become Hindu. Ask the father what she should do to become a Hindu. Is it enough if she says “from now onwards I am a Hindu”? They can perfectly continue as a Hindu couple, except that they should not tell their antecedents and try to enter Lord Jagannath Temple at Puri. Is it not possible to live in this world without going to Jagannath Temple? Do not enter Mookambika temple in Kerala. But if you must, enter incognito. If caught offer to pay Rs.25000/- for purification. Or privately pay Rs.1000/- to the priest and close the issue.
Why all this? Just follow the example of my neighbour. It is a real story not a reel story. I can give you names and addresses.
One strong advice: Never consult lawyers for these things. They will search for acts, sections and clauses.