Yes, I had to do a bit of Googling, so if any of the details are not quite right, that's my bad.
Tbf, only 2 countries have ever left (Greenland and UK). And then you have countries like Norway and Switzerland that aren't members but have their own bespoke deals and various levels of integration and access to the EU such as the Single Market and freedom of movement. I'm curious about the punishments you've mentioned for countries that leave. UK has lost access to a lot of schemes and programmes and mutual agreements such as on sharing intelligence, but that was already accepted by leave voters as the price of leaving.
(If you're not a member, you don't have those "privileges".) I'm not aware the EU has put up significant trade barriers with the UK like Trump has done, or even desires to because it would arguably harm them too, other than the tariffs that apply by default to any non-member. The UK government also negotiated their terms in the TCA (Trade and Cooperation Agreement) they signed in 2021, and opted for a less closely-aligned arrangement to start with out of various possible scenarios (and which current PM Starmer has recently started slightly softening, which is being hotly debated right now).
I'm not quite sure about the opening the borders to Islam thing. 🤔 If I'm not mistaken, the majority of Muslims (or their descendants) in the UK have roots in South Asia (India, Pakistan, Bangladesh, etc) and there is a sizeable minority of Muslims with Arab or African roots. But those are separate to the EU, and mostly from ex-British colonies/Commonwealth countries rather than EU countries. EU immigration included a percentage of Muslims, yes, but I think that was a minority of the total. Immigration from Commonwealth countries (and net immigration in general) has actually increased quite significantly post Brexit, while it's dropped off from EU countries, as was to be expected.
There is still the ongoing "migrant crisis" which is a problem that requires a solution for all involved. However, irregular arrivals (including Channel crossings) make up a small percentage of total net UK migration (around 5-7%). ~40K out of ~700K+ total net in 2024.
As for Sweden having a religious civil war, well, let's hope it doesn't ever come to that.
https://www.gov.uk/government/statistics/irregular-migration-to-the-uk-year-ending-march-2024/irregular-migration-to-the-uk-year-ending-march-2024#how-many-people-were-detected-arriving-by-small-boats