I would do it that way because a model is one object of model series and a model series is one object of manufacturer.
It also removes the duplication of data. If you need the manufacturer of a model you can get that information from the model series it is part of.
Yes, I would use bidirectional relationship fields.
The plugin you mention is mine.
Yes, the fields that connect each item would have both in their names, but I would probably use something short and simple. What you would use would depend on how you generally name thing.
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