Re: Visiting multiple Schengen countries
You can be dismissed at the border of an other Schengen country inside the Schengen area.
There are no borders inside the Schengen area, that's the whole idea. The other points you quote relate to moving from a non-Schengen EU country to a country within the Schengen areas. As the text you quotes says "of the country whose external borders you will cross first when entering the Schengen area".
You only need a single Schengen visa to enter the Schengen area. We see this a lot with US visitors, the ones who travel on US passports can use visa-waiver rules to go to any EU country, but those with, say, Indian passports have to apply for one visa for all Schengen countries, and separate visas for each non-Schengen country (like the UK or Ireland) which they also want to visit
Visa permit is going to be required for EU citizen to enter UK from 1st November 2019. I don't know what the visa policy of the UK is going to be for people from EU/EEA. If they have any visa policy. I am not sure they have a visa policy.
They have, see https://www.gov.uk/visit-europe-brexit "Visas: you should not need one for short trips. After Brexit, you will not need a visa for short trips, according to European Commission proposals", and if the EU doesn't impose a visa on UK citizens there's no reason for the UK to require one for EU citizens. Most countries work on that sort of reciprocal arrangement, UK passport holders can travel to 183 countries without a visa. I think the "best" passport for that is a German one which allows visa-free travel to 187 countries. When ETIAS comes into effect in 2021 it will probably affect UK citizens, who will need pre-approval (not quite the same as a visa).