You are right. I checked the processing time - it now takes 82 days to process visa extention. That is almost 3 months.
But the implied status is still there on their manual. See attached.
The web says at least 30 days - true, that is strongly recommended. Whenever possible, apply 30 days before it expires. Even that, you will still have to have 52 days of "implied status". Not everyone is comfortable with an implied status and having no document to prove their legal status in Canada.
Your understanding is right. The expire date on the visa is the date you have to enter Canada. Once you enter, you should be able to stay for 6 months, unless the officer at the border had written something like "one month" or "three months" on your passport. Default is 6 months.
The officer will not stamp an expire date on your passport. If any, there should be a stamp of the date that you entered Canada. Some countries do give you two stamps on passport. One is the entry date the other is the date you must exit, like US. But as far as I know, Canadian immigration officers at the border only give you one stame, the date of entry.