There is no reason for Dash to implement segwit right now, there are certain risks with segwit (which i imagine is why Bitcoin Cash has not implemented it)
and it is a very temporary method of getting some breathing space for a cryptocurrency which has to deal with full blocks. Dash does not have full blocks anyways,
Dash blocks are rarely totally full.

InstantSend enables Dash to have its transactions confirmed instantly and also locked to prevent double spending. It is an optional choise for users and there are some
additional fees involved. It does not really relate to segwit, i think.