Here’s the sentence I was making:
Können Sie jemanden in München empfehlen, der diesen Wasserfilter installieren kann?
For some reason, the Hauptsatz doesn’t feel right to me – is it?
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Technically speaking your sentence is ok, but I as a german would say:
Können Sie mir jemanden in München empfehlen, der diesen/dieses Wasserfilter installieren kann?
I think you missed 'mir' as a personal pronoun to which person someone should recommend something. This makes the sentence more clear and fluent to speak.
Yes, the main clause of a relative clause follows the same word-order rules it would follow if the relative (or less specifically: the subordinate clause) were not there.
So always build a main clause according to V2 (verb in second position) unless it is an imperative or a question. Your example sentence is a question, so the verb coming first is perfect.
If you’re interested what the non-question form of the sentence would be:
Sie können mir jemanden in München empfehlen, der diesen Wasserfilter installieren kann.
There is one tiny catch you need to be aware of: If the sentence starts with the subordinate clause, that is considered to be the first fragment of the main clause and the verb must come second.
Wer den Wasserfilter installieren kann, verdient eine Empfehlung.