Which tense should this be? "are to" vs "were to"

The subjunctive were is used for a.) counterfactuals:

• If I were rich... (but I'm not rich).

Your text contains one example of the counterfactual in the sentence:

• If it were not clear that she is determined to move away from this area... (but it is clear that she wants to move away).

It is also used for b.) predictions about the future that the speaker believes are unlikely:

• If it were to snow tomorrow... (but it's high summer).

By this token, the words Assuming we were to lose her... can be regarded as implying a doubt that she will leave. But this doubt seems unjustified in view of the fact that she is 'determined to move away' and has applied for a job at another company.

In such a case the subjunctive were does not seem as 'correct' as the present tense (or Conditional 1, if you prefer to use these problematic terms). Nevertheless, there is nothing ungrammatical about were in the sentence.