You are confusing trade deals with business deals. Business deals/trading will continue, its the terms of the trade that will alter, and even then may well continue albeit at a higher cost.
Tit-4-tat applies to both sides, that's what tit-4-tat means. If the EU applies a tariff, the UK can apply a tariff..... tit-4-tat. And the EU is a member of WTO - there's even a German finance minister on the WTO board.
The deal that May brought back wasn't accepted, as you know, and that includes the £39bn. Its listed very early in the 585 pages. The EU can jump up and down as much as they like. Nowhere in the EU Articles is post a country leaving is there a financial commitment. Imagine a country leaving that was a net receiver. Would the EU continue sending money? Don't be silly.
As for roll over deals; why roll over a trade deal that currently includes tariffs and quotas? You need to stop applying today's EU trade rules the UK has to post-Leave. And as the UK hasn't left it can't yet offer different terms. Have you learnt anything in the last 3 years?
Have a look at business financial passporting. The EU already recognises that it wouldn't be wise to hinder investment coming from the UK.... anyway, have a look at it for the detail of what has already been agreed for business investment - note, already agreed. Personal banking financial passporting is an issue - god knows how my pension will get to Spain.
