As a greeting or introduction, it’s always “welcome to” rather than “welcome at” or “welcome on” or any other such preposition.
Nothing tricky about this one. A role or profession in the singular requires an article: “he is a teacher” or “she is the CFO”.
It’s “at”, not “on”. Negotiations at the European level. Coronavirus cases still at a high level. Figures at the level of the individual business units.
No, I’m afraid you don’t. You live on Bovenweg. There’s no article needed for a named street or road or square.
There’s no such thing as “a training”. You either receive training – a general, uncountable noun – or take a training course.
Whether to use “a” or “an” depends on if a vowel follows. But remember: that’s determined by the spoken sound, not the alphabetical letter.
Nobody has an insurance.