Well this certainly gives me a better understanding of your status. Your work exp is more than 1yr but less than 2 yrs with or without the 5mnths self-employed, hence I would suggest you completely leave the 5months out else it'll only be a hassle for you to gather all the paperwork as there are no additional pts for 1.5yr of work exp. Better to leave things out than to be deemed as misrepresentation and panic.
As far as the job title goes, you can mention both the titles in the reference letter since you worked for the same company. It's the Duties/Roles & Responsibilities that CIC looks for. Both NOC 2171 & 2173 would fall under the same category according to this link:
hrsdc.gc.ca/NOC/English/NOC/2011/ViewAllTitlesQuickSearch.aspx?val=2&val1=2171&val65=2171
If the HR can form a link between the duties you perform and reference the HRSDC website then yea, it would be a safer option, else don't worry about it, just a title should be enough. Even better, if you can draft your own sample reference letter according to CIC and then have the HR sign it and print it on a letter head.
Considering your score, I can say for sure that you will get an ITA soon, I still have to bump up my score. Good Luck