1, The issue of GM is not confined to crops, that's your interpretation. I cited "Plants, livestock and people". Now, I don't know enough about genetic medicine to be sure of the size of the opportunity, but I take the word of my wife who is qualified in that field and she thinks it is the future of so much medical development.
In any case, the growing of crops is not necessarily the issue, but the development of the plants, which requires little land may be where its at.
2. While I don't subscribe to the view that the economy is the most important aspect of Brexit, you asked for an example to which I responded. But I did give you an example from the economy --- fiscal convergence-- which provides a potential game changer. We will be able immediately to set our own VAT rates and sales tax regime. This could rescue the high street and provide selective boosts to chosen areas. Boris has said that on day one we will zero rate feminine hygiene products, which EU refused us permission to do. (A small but important issue). If fiscal union or at least steady convergence comes about, and I am sure it will, that will be enormously damaging to some EU countries, but we will escape that damage. The days of tax arbitrage are coming to an end.
3. In general terms, the thing to do is to look to the possible future EU policies to gauge whether Brexit will be an advantage or not.
Edit PS EXAMPLE. If Scotland gains independance and joins the EU, how will their citizens react to tax rates: Vat, income tax etc, much higher than they enjoy now?