I’ll try to answer in Bens stead.
To summarize, I think you will have to test for yourself to get the correct answers to your questions.
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What have you tried so far? I would try with the examples posted in the linked thread and then take it from there. It is basically impossible for anyone but you to know what will work in your specific setup. Take care to keep, and possibly update, any extra packages that may have been added in your current composer.json. Also take care to read all the posts in the thread as there are some questions and answers in it. FWIW, I have managed to upgrade a couple of Sage9-sites using the linked thread as a starting point but I always had to make adjustments to fit my needs and conditions.
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Since 8.2 is >=7.1, you probably don’t have to change this. But I would do it anyway and set it to >=8.2 if that is what you want to run under. If this does not work, you may have to change it back. There are some posts in the linked thread about this as well.
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If the remote server is running the same PHP-version as your dev-environment and it works in dev, it should work on remote as well, yes. But there may be other things that come into play and I would not recommend updating PHP on a prod server/environment and push the changes before you try it on a staging server/environment with the same setup as you intend to update prod to. If the prod environment is not business critical, by all means, go ahead and update it without testing first. It’s up to you
Also, please read How to best ask questions on this forum and especially about formatting code and avoiding screenshots.