Is it permissible for the groom to deliver the marriage sermon (khutbat al-nikah) and to conclude, write, and present the proposal himself in the presence of the guardian (wali) and family members? And is there a similar example of this in the hadith or other religious texts?

The marriage sermon (khutbat al-nikah) before the contract is a Sunnah. It is recommended for the contracting party or someone else to deliver it using the sermon of Ibn Mas'ud. There is no harm in the prospective groom himself delivering this sermon and addressing the bride's guardian. This is the original principle. As for writing in official documents, it is not a condition for the validity of the contract; rather, it is for documenting what has occurred and safeguarding rights.