You are correct trade will continue and changes will be made by customers/suppliers to adapt to the new market conditions. You are also correct in citing the problems for the larger/multi-national companies, that the loss of frictionless trade will be the big problem and if companies do intend to relocate to get around it, then its possible some of that relocation could be from the EU to the UK, although I accept most will go the other way!
The fact is nobody knows, except for some immediate dislocations, just how things will work out in the longer term. Change is always frowned upon, worried about etc. even by those who may ultimately benefit from it. As with any period of rapid change, the fittest will survive the weak go to the wall. The fact that for many it will be seen to be self-induced is what makes it worse. The heart felt plea from many remain voters seems to be " my future is being decided for me, against my will, when do I get my say"? Which is probably precisely what Leavers were thinking in 2016?
The only way the EU will change course over Ireland, is if Ireland says so!