Connecting at YYZ, but not sold as connection
 

Flying YUL-YYZ-FCO. This is 2 different flights booked. 1 from YUL-YYZ and the other YYZ-FCO. There is 5 hours between.

Do we think Air Canada can check the bag through or is it safer just to pick up the bag and recheck it?

Note: Flying J, so no baggage fees anyways.

Check through.

Yup present both tickets at check-in at YUL, and they'll be attached, your bag checked through, and your BP's issued all the way to FCO.
You might even have a case to get the YYZ AIF refunded down to the $4 "connecting" level rather than the $25 departing fee by sending in both tickets to refund services after the flight with an appropriate explanation.

Another interesting question - will OP get the J dom -> int'l connection service?

Sorry for my ignorance, what exactly is that 'service'?

No. Aeroplan awards. no BMW.

From what I've read, you have to be REVENU International Signature Service only to access the signature suite and the BMW transfers.

Uhmm, my experience is transfer by traditional method; walking.
2 TPAC connections through YYZ under the auspices of Signature Service.
Not complaining though as the walk is good for me. A YYZ TPAC for me is usually 10-12 hours of being crammed under the painful B777 table. I never know if I might get on a flight with Kit Kats or the rare Lindt, so I need to burn a few calories, just in case.

Who would qualify for limo service? I am thinking SE paid J fares?

Anybody who ponies up the money for a JCDZP ticket. Not limited by status.

That's nice, but it isn't the case right now. It is a rarity to be offered a transfer. AC is upfront and honest too with its disclaimer
(my bolding & colouring)
Later this year, Air Canada Signature Class customers will be offered Air Canada Valet Service, powered by BMW across the tarmac when connecting from a domestic to an international flight at Toronto Pearson.

My father is planning on fly the following

PVG - YYZ - MCO
MCO - YYZ - CDG

due to obvious reasons we could not buy them via AC as O/W EU is literal robbery

PVG - YYZ will be J
YYZ - CDG will be PY(most likely upgraded to J)

Can he connect in YYZ for both cases?

What are you trying to do there? Ditch the YYZ-MCO-YYZ flights, and merely connect at YYZ? I don't really understand what you're asking.