Wife and I will be transiting in YVR from USA to Europe in a few days. We'll have a long layover and we're planning to leave the airport and go into the city center (we're eligble). I need your help for a few questions:

Question 1: is it feasible to go from transborder section to international section, go into the MLL and have breakfast (we're eligble), then leave the airport to the city? Or would it be more reasonable to leave straight away for the city center?

Question 2: is there any place we can store our luggage at the airport, before going to the city center? Is it possible at the lounge?

Question 3: any trouble we should antecipate in getting to the international area again? Some airports in Europe are a zoo at the moment, with long queues even for J or status holding passangers. We are planning to be back at the airport 2h before our flight.

Thanks in advance

Unless something's changed in the past few years (like how YYC basically tore down the wall demarcating their "international" terminal), it's not very easy at all to exit the international gates at YVR. I've only ever done it once on a ticket that I had to:
(1) cancel check-in,
(2) get the MLL lady to offload me from the flight,
(3) get her colleague to call CBSA so that I could re-enter Canada,
(3a) that colleague is satisfied that I'm truly not flying today and asking me why,
(4) CBSA is satisfied I didn't buy anything new at the gates since I technically don't have a duty-free exemption, but they have to do the entire declaration/cancel ex-Canada record over the radio since I'm not at the typical border facility,
(5) re-enter landside via some service hallway

And all this was for travel originating from Canada, it was very inconvenient. I think you'll have a much harder time trying to stay on your flight but entering Canada from the part of the airport that isn't designed for it simply because you wanted breakfast...sorry

It'll be much easier for your to enter Canada the regular way and forget about the international terminal until it's time for you to return and enjoy it. There's decent brunch and dim sum within minutes of the airport via public transport, you'd be missing out at the MLL anyway

Yes there's baggage storage

You don't arrive at Transborder.

I lately originate at YYJ, but from US you either enter Canada, or go through some sort of control to Int'l terminal - where there is a physical clear barrier, so the two don't mix. My last connection was pre Covid, so it's possible it's changed, but I would be VERY surprised.

As noted, yes, there is storage at YVR - and no, not at any of the lounges.

When I left Canada a couple of months ago there were LONG delays at YVR (and YYZ) due to staff shorgare, and I doubt it's improved since. Be prepared for two hours to get through security unless traveling early or very late.

You will need to clear customs.
Luggage storage is level 2 domestic arrivals.
May not be easy to find, but it's near carousel 5 or 6.
From there up to level 4 to catch the train into the city.
Buying a daypass is probably the best value.

I would just not worry about MLL and head straight into city.