Your deduction of title clause is fine. The only change Inwould make is to add details of your property after the words "County of Selkirk" by adding the words "over All and Whole the subjects known as and forming" and then copy the address and conveyancing description from the Standard Security. I hope at helps. Please leave a positive rating so that I am credited for my time.
If your former address appears on the Standard Security it is good practice to pick that up in the Discharge. And the proposed wording of your covering letter is fine.
No you need to put in a testing clause yourself if you are the presenter of the deed. You can assume the witness's address is the address on the letter returning the discharge.