"Have good weekends" vs "Have a good weekend"

Solution 1:

Was this grammatically accurate and valid greeting in English? If not, what would be a better way to convey his message?

Technically "Bye. Have good weekends" is correct in that he is wishing each individual a good weekend, using the collective noun for all of your weekends. However, colloquially this strikes me as lazy grammar. Expansions would be more like:

• Bye, I hope each of you has a good weekend
• Bye, I hope you all have a good weekend

The reason I would use the longer version is that it is more personal to each team member, the blanket "have good weekends" feels like more of a throw away comment such as "bye, I'm off".

is anything grammatically wrong with the ambiguity of this statement

No one would refer to "several good weekends" in this context and so I would not say this is ambiguous, it is clear he is referring to a group of people who each will experience a weekend. There is nothing particularly wrong with the statement, it is just lazy.

Solution 2:

It depends on your definition o "weekend". The time allotted between Friday at the end of work to Monday start of work is technically correct.

However, when you come back to work on Monday, a frequently asked question is, "How was your weekend?" That indicates each person has a different weekend, even though everyone shared the same time span.

I would say, personally, "weekend" because I recognize it hasn't happened, yet, so currently the individual weekends of each person have not occurred, yet.

If several people told me of their plans for the weekend, their weekends exist, if only in the idea stage. In this case, I would say, "weekends".