Finally, we got the confirmation for D visa few days ago.

Here are some details might help somebody else with same situations.

Regarding rental contract : last April my fiancé found an apartment in brügg that is available in June/2019. She applied and have requested in the application that we would like to start renting in JULY/2019. she received a phone call from the landlord asking if it is possible to start renting in june/2019, she said No. Finally they accept renting starting from July and she signed the contract in May.

Which migration office she dealt with :
Migration of Solothurn only despite that she was told by a migration officer that they may send the file to Bern but that did not happen.

Regarding financial sponsorship :

They accept her father as our financial sponsor.