With respect you ask so many questions that you would be very ill-advised to even contemplate completing the form yourself. After all, SDLT is a self-assessment tax and if you get it wrong then the consequences will cost you far more than the £50 the the solicitor envisages charging for ts completion.
Also, you are taking out a mortgage in order to proceed with the purchase? Consider this. It will not be possible to register the property transfer at the Land Registry until after the Inland Revenue issue their certificate, and they will not issue their certificate until after the SDLT return has been sent in and the tax paid. That is, the mortgage will not be registered at the Land Registry until after the form is done and sent in ... and therefore the solicitor acting for the mortgage company (bank/building society/whoever) may insist that the form is filled-in professionally. Or at the very least, that it is completed on a timely basis .... and checked before submission to Netherton ... and how much will someone charge to do that checking?