Staying in UK six months?

The UK is simple, if you meet the requirements for entry, you'll be allowed in. If you don't, you won't. So if it were me, I'd get hold of the list of requirements, and make sure I met them, and have the required supporting paperwork. Not sure why you are panicking so much about something that is so defined.

Maybe I should apply for the visitor's visa in advance?

Do the requirements say you need one or not? You seem to be guessing.

Have you talked to the relevant authorities for some advice?

I'm not unsympathetic to your concerns, but you do seem to be worrying unduly.

You as a Canadian citizen do not need a visa to travel as a tourist or to visit family and friends. You should however bring supporting documents, the same as you would if applying for a visa, to be presented if asked for. This is an easy google.
Please provide an example of these horror stories where compliance, as required by a Canadian has failed to give access to the U.K. (and which I have never seen).
Your worries are unfounded.

You will probably not need the "letter from the friend taking care of my pets".

Questions for you to ponder (to yourself, not out in public):

do you have a criminal record, either in Canada or the UK?

will you be importing any contraband?

have you been refused entry anywhere else?

have you been to the UK often enough that a pattern can be seen, or a determination to overstay?

or a few other esoteric questions...

If no to the above, and if you can get the "letter from the friend taking care of my pets" you should be OK.

The 'horror' stories are mainly by people that bring up the immigration flags relating to potential over-stayers and workers. Such things as not having details of a departure flight, not knowing where they are staying, having little money or access to it, no ties back to home, 'volunteering' not for a recognised charity, having copies of their CV/business card/tools of the trade etc. Plus visiting a boy/girlfriend.

You have the right documents, have money, have financial ties in Canada through a job etc. So even though Canada might be high in the list of countries for refusals that doesn't mean that many people are admitted without trouble at all.

(For those interested the top ten list for refusals into the UK varies a bit from year to year but typically goes something like:
1) USA
2) Brazil
3) Albania
4) Nigeria
5) Canada
6) India
7) Australia
8) Pakistan
9) Ukraine
10) Malaysia

However, don't have nightmares about this.)

Another flag is people with multiple entries within a relatively short period of time (ie: playing the Schengen shuffle), but you do not meet that criteria unless you have visited them several times. Seeing that you have family, have not visited in some time, and can show some documentation, go for it.

I would not push your six months to the day, leave yourself a safety buffer.

Thanks for responding, everyone!

I should have probably mentioned why I'm concerned:
- My YMS visa expired in February. I left within the appropriate time frame, so that's not a problem.
- My month-long visit was in June.

In theory, I'd be going over there in early December, so that I could spend Christmas with them, and then staying until late May, which would be nearly six months but not quite.

It's the repeated visits that have me worried, is the thing. I have zero intention of working or overstaying - I just want to spend time with my parents and be a tourist, since I wasn't able to do much tourism during my YMS stay. However, I'm worried that the repeated visits would be a red flag and that they won't believe me.

The rough rule of thumb when repeated visits may raise a flag is any 6 months in 12 but repeated visits over a longer time can be looked into. It sounds as you might be reaching towards that kind of territory. However, your documentation seems sound as you describe it. You could reduce your planned stay down from the maximum too I suppose.

I really think you are overthinking this. But then why not contact the British Consulate nearest you and just ask what documents you need? Don't go into great detail and say you are worrying, just that you are visiting and living with your parents who are currently working in the UK and what documentation should you have with you. Sounds like you have most if it now (letter from parents, boss, return ticket and bank statements).

When my Mum was still alive I was backwards and forwards to the UK three or four times in a year (on my Canadian passport) and had no problems - mind I was staying only for a couple of weeks at a time. You are more likely to raise a red flag by appearing stressed and nervous. One suggestion, make sure you passport is good for six months after your anticipated return to Canada; I remember once going when my passport was due to expire a day or two after I got back to Canada and I was given some stern reminders from both UK and Canadian Immigration!

Good luck and enjoy the chance of living and traveling in the UK for an extended period.