I can understand the political arguments against the EU even though I don't share them. The economic arguments for Brexit seem flimsy at best.
Let me question your first point that Brexiteers were looking at building an arrangement with China as an alternative to European trade..nonesense! I have not heard anyone making the case that in some way the UK's exit would mean that we no longer trade with europe and then build relations with other countries. We would continue to trade with Europe after exit, in much the same way as we do today, and of course we would continue to trade with the rest of the world as we do today. However, we would be in a better position to protect our home industries if we need to, for example we could quickly put tariffs on Chinese steel should they be dumping cheap steel into our market, or for example choose to import cheaper food from the rest of he world free from European tariffs designed to protect, for example, FRench or German farmers or agri industries.
If we are going to talk flimsy economics, then you need look no further than the cornerstone of further European integration, that is the Euro. The whole European economy (including the UK's) currently, and over the last five years, has a drag on growth from the inadequacies of the Euro; the European Central bank policies; and the flimsy performance of the weak banking systems and economies themselves of a number of the constituent national members of the Eurozone. It might be a fantastic currency to replace a multitude of currencies operating in a single closed market, but it is not so efficient when dealing with disparate national economic performances; differing national fiscal policies, and in some cases tax evasion on a huge scale. Never mind the sevre economic problems we have seen it produce in Greece, Portugal, Ireland, and indeed in France and Italy. It might be a big world currency, but I would judge that it is smaller as a whole than its constituent national parts, simply because of these and other inefficiencies. And given the EU (and the European Central Bank's) and their unaccountable political elite's poor record on dealing with any number of issues around economic performance and other issues such as security or agriculture or industrial policies I can only see the Euro to be a continuing millstone around the neck of the EU in particular for years to come.