"each country, through its EU commissioner, gets one vote and the power to veto"In theory yes. In practice not really. The EU has a long term goal of getting rid of veto and there's hardly anything left of it now. Also, even when votes take place they are often secret and there's no way to know how countries voted. Also the countries are supposed to nominate whoever they want as commissioners, but Juncker talked about how in reality there's a secret veto and the head of the Commission ensures they only get the Commissioners they want. We can infer that they get commissioners who are on board with their agenda and more loyal to the EU than their host nation.
To ensure that loyalty they have a lot of other tricks up their sleeve, like enormous "pensions" that are contingent on continuing to demonstrate loyalty to the EU after leaving the job i.e. they're not really pensions, they're bribes. They also pay much better than host nations can, and have a special tax deal such that they pay virtually no tax at all so their comp at the EU is sky-high compared to what they could get working for their own government.
So national control over the EU is really extremely weak in practice.
"The things the EU commission can legislate are restricted by treaty, I've no idea what these restrictions are"
Again in theory anything not delegated to the EU is controlled by the nations. In practice, again, that's not really true. The EU Commission is a master of reinterpreting the treaties to mean whatever they want, backed by the ECJ. For example corporate taxation is not delegated to the Commission. They fixed this by re-interpreting their powers to stop state subsidies as a general control over all corporate tax rates, by arguing that a tax rate lower than e.g. France was the same thing as a subsidy. Normal people know that a tax is not the same thing as a subsidy, but the ECJ disagreed and thus the Commission took control over corporate taxation across the EU without any treaty change.
That story has been replicated numerous times. Regardless of what the treaty authors thought the treaties said, the court is packed with judges who are ideologically committed to the EU as a project, and who will happily engage in judicial activism to extend its powers.