I'm not sure the marriage route applies now. Someone I know in Britain married a woman from Eastern Europe who had lived in Britain for several years but, some years after the wedding, she still had to apply for British citizenship. It didn't happen automatically upon marriage.
Double taxation is a vexed question. The country in which the person resides feels that the person ought to pay towards the services provided. The country of which the person is a citizen feels that the person ought to pay towards the services it provides. In the latter case, you may ask "what services?" so let me give an example. Should a US citizen resident abroad get involved in some kind of difficulty abroad (from losing a passport to getting caught up in political unrest), he or she would expect the US government to help - consular services perhaps or rescue even. The provision of those services cost money that is raised through taxation. It is not a straightforward matter.
On reflection, I think Google's European base is in Dublin. U2 moved their base from Dublin to the Netherlands and Google settled on Dublin. It sure is a mixed-up world
To move on, these various countries are not known as "tax havens" without reason. THE ECONOMIST, a well-respected and prestigious UK magazine, has adopted the following definition: "What ... identifies an area as a tax haven is the existence of a composite tax structure established deliberately to take advantage of, and exploit, a worldwide demand for opportunities to engage in tax avoidance."
It must have been 50 years ago that I bought a copy of Andre Hodeir's "Jazz : Its Evolution and Essence". My copy is a paperback, with now-yellowed pages, many of them loose, and I forget almost all of its content but I do recall two of the epigrams it contains. One was from Freud: "All attempts at definition give rise to difficulty; let us therefore not hope to escape them in our present effort". THE ECONOMIST acknowledged that the above definition of a tax haven is not without its difficulties but it is a start and, just because it is difficult does not mean that we should give up on refining it.
The other epigram I recall from Hodeir's book was from Camus: " .. where lucidity reigns, the scale of values becomes useless". Many of these tax havens offer secrecy, whereas transparency ("lucidity") should be the rule. If these tax arrangements are legitimate and comply with the appropriate tax legislation, what do these people and corporate entities (especially the corporate entities) have to hide? To reverse Camus' words, "where lucidity does not reign, a scale of values becomes necessary" - very necessary in my opinion.