According to the Canada.ca website you can only file through Netfile for 2014-2017. This means that the previous year returns can be completed in the corresponding program for that year but would have to be printed and mailed in. Do not just send your T4/other tax slip to CRA they likely already have it as your employers are supposed to send it to CRA. The important part is completing the taxes and including any deductions you may qualify for.

Regarding the HST credit: If you are eligible for the credit and entitled based on your income and family status it would first be used to reduce any balances owing (whether from these tax returns, prior taxes, or other government debts like student loans or family orders if you are above the financial hardship level ~$20,000 call CRA for more information). If you are not owing any amounts and you are eligible for the programs and entitled to amounts then they should be paid after your tax returns are completed. As to how much you may get u/pigpong is correct that depends on the final values on the return. Assuming you have all information for all years this may give you an estimate.