～とまではいかないが/とまではいかないとしても is a construct that means something to the effect of "is not that much, but...", "hasn't reached that degree, but..." I remembered it being a JLPT grammar point, but strangely I couldn't find much information on other sites. But please see this related Q&A: とまではいかないまでも meaning
Here いく can be roughly understood as "to reach (a degree)". So the preceding thing is taken to be a baseline for comparison, and the thing being described, often the sentence topic, has not reached that baseline in degree/intensity/significance etc. This construct sometimes corresponds neatly with the English phrase I wouldn't go as far as to.
"I wouldn't go as far as to call that a present from Santa". Of course in the Japanese construct it is the thing at issue that does the "go" action, whereas the English phrase has the speaker inserting their own agency into the utterance and doing the "go", but semantically they could both arrive at the same meaning. Essentially this is hedging language. The speaker is comparing one thing to another, but is unwilling to assert that they are the same.
で is short for それで/そこで, meaning "then". A conjunction (接続詞) in this sentence, it actually makes sense here, more so than if it were であれば. I'd expect something before であれば.
I take あれば to mean (もしそれが)あれば, and それ possibly referring to 恋人になること.