Hi ametisto (Hi Ron )
In reply to your first question, 'is shifted' doesn't work but 'shifted' or 'has shifted' does. The latter also follows the tense in the first sentence:
In Germany, the first of these two sources has run dry due to demographic reasons. As a result of that, the second source is also receding since investments are on the decline and production [has] shifted overseas. Note that, neither of the sentence above is written in passive voice.
In reply to your second question, the subject of the phrase 'the higher number of immigrants' is not the word 'immigrants=who', but rather the quantifier 'number-=which':
... the high number of immigrants, the best part of who pertain to the above age group, doesn’t manage to bring this decline to a halt. Note, 'the better part of' is a collocation.