If I give dowry without involving the Shaikh to officiate, is the marriage valid?

A Nikah is performed with a proposal (Iejaab) by the male or female and acceptance (Qubool) by the male or female in the past and/or present tense in the presence of two male Muslim witnesses (Hidaaya vol. 2).

It is Sunnat that the marriage be announced and performed in the Masjid and the bride be represented by her Mahram (father, brother, etc.). The bride gives consent to her representative (Wakeel) in the presence of two witnesses to perform her marriage at the Masjid. At the Masjid, the Wakeel represents the bride in the presence of the two witnesses who must be trustworthy and pious male Muslims. They should not be from among the girls ascendants e.g. father, etc. nor from her descendants, e.g. son, etc.

Having said that, if the sister (attained puberty) wishes to represent herself at the time and place of Nikah, then she may do so without a Wakeel. This is according to the Hanafi Madhab - according to Shaaf'ee permission is incumbent.

A marriage without the two witnesses is not valid. the relationship will be illegal and Haraam.

and Allah Ta'ala Knows Best
Mufti Ebrahim Desai