Namasakara - saying it right

The word नमस्कारः or नमस्काराः is a very widely used in most of the Indian languages. In those languages, it is used like: नमस्कारः to you, नमस्काराः to you all. So, when we want to use such a common salutation in a Sanskrit sentence, it sounds straightforward and logical to form a sentence as सर्वेभ्यः नमस्काराः - because सर्वेभ्यः means “to all”.

However, the usage in Sanskrit does not go like that. The correct usage is सर्वेषां नमस्कारः. If that is translated word-to-word, it becomes “नमस्कारः of all” - which sounds weird. That is the reason, word-to-word translation (for any to any language) should not be relied upon.

In Sanskrit literature, this word नमस्कारः is almost always used in stand alone mode, not with any other word in a sentence. What that means is: in Sanskrit, think of the word नमस्कारः as a general statement, (literally meaning “the act of bowing”) and not directed to any particular person or group of people. So, it is better to say just नमस्कारः. But if you cannot resist adding “to all”, use it as सर्वेषां नमस्कारः.

Such use of नमस्कारः with another word in a sentence is extremely rare in Sanskrit. One example can be quoted from the drama वेणीसंहारम् (of भट्टनारायणकविः) - संजय, पित्रोर्नमस्कृतिं श्रावय - Here पित्रोः is used in षष्ठी विभक्तिः, same as that of सर्वेषाम्. Panini's grammar also supports the use of षष्ठी विभक्तिः in such cases. (Ref: कर्तृकर्मणोः कृति २.३.६५). Per that, सर्वेषां नमस्कारः (and also सर्वान् नमस्करोमि as a verb) is correct.

Bottom line:
It is better to use नमस्कारः or नमस्काराः as stand alone words. To address others, stick with नमः सर्वेभ्यः or नमस्ते.