Was already mentioned Tulsi bro checked (part of) the letters with Tulsi mom,she had no doubt were written by her daughter.
For drama ghost threw letters aside while Gungun was sleeping but never thought to destroy or replace with a version where D would be the culprit.
That may be because T is NOT sure who set her room on fire to burn her alive years back,😳 or ghost is a hopeless fool when it comes to trapping D but she can easily trust an obsessive hypocrite stalker R to trap M into marriage.
May be, like some of us T also feels D didn't kill T directly otherwise, ghost could have got some powers to kill D long long back & take D with her to ghost land..🤗