How should I insist to be the first author?
Insist may be the wrong approach. Given your #3 and #4 above, he may have a valid claim on it. You can ask, of course, and you can appeal to his &quot;better nature&quot; that it will boost your future prospects. That might be a win.
But if you balance off his good will and support against first authorship of a single paper it may turn to your profit to yield if he insists. Your first work, especially at the masters level, will, hopefully not be your best. It is a plus in your column no matter where you land in the authorship order. And if you trade it for a better and more enthusiastic letter of recommendation for your next steps it may be worth some sacrifice.
But there is a bit more to it here, since this is a conference paper and hopefully the conference will give you a chance to expand your circle of contacts. But that, too, might depend on having his good will.
Think strategically and for the long term. Don't compromise long term benefit for a short term &quot;win&quot;.