I find for race day that it is OK if I allow three hours before the race. Maybe you can get away with that. As for the second part, I religeously have a sachet of instant porridge three hours before the race, regardless of what any particular hotel might offer for breakfast. I follow that with half a litre of sports drink 2 hours before, so I have time to pee out excess liquid before the race. Maybe you could trial a similar regime.
According to web searches it is likely you can tolerate porridge so long as it is not contaminated with wheat in the mill. Some brands might offer this, e.g., from healthfood stores.
Have you tried using the second lace hole for the final cross? You kind of make a loop and pass the other side through the loop and then tighten the laces as usual. It's a way to reduce heel slop. Maybe that would help. Or maybe an extra volume adjuster insole from a walking or hiking shop would help.
I don't think the sizing as such would be the problem - you could have a touch of the classic runner's knee. This article is pretty up to date and reflects current thinking. See if it makes sense for you.
Presumably you can walk without shin pain? Can you walk in your new running shoes without shin pain? Maybe you have bought some new shoes that are flatter (low "drop" or difference from heel to toe) and this is making you feel you have to hold up the front of your foot with your shins, or your achilles is holding your foot back. Whatever, you need to find a simple programme to transition into running. Try googling for a "couch to 5 km" programme or google the walk-run method and somehow plan an extension of what you can do now without pain.