I used a strip of mild steel for my robot in a similar way as you plan to here. It was 4mm and stood up perfectly well to non-spinners. Personally I wouldn't go less than 3mm. If you used 2mm you'd need several spares and could be spending lots of time straightening it out after matches. Is the front of your lifter touching the ground without this addition?

Not sure if you'd get away with multiple little bits for a cradle - if you are tight on time you don't really have to build much though. My carrying cradle is just a thick, old wooden chopping board with big cut outs for the wheel area, so it looks like an I shape. If you have any chunky HDPE sheets leftover that you could fit between the wheels that would work too - so long as it's wide/long enough and the robot can't tip off it.

To be on the safe side I would put some sort of locking bar on - due to the nature of your design it shouldn't be too difficult.

Also what weight are your wheels? I personally haven't tried this yet, but I think at Insomnia there was several robots which had 4 holes drilled into the wheels in a square/diamond formation to cut down weight. Maybe try as a last resort?

Barrog is looking great by the way - nice work!