Even if I grant your version of the story, the overarching theme of Judges is "every man did what was right in his own eyes", people were doing messed up things and the Bible does not comment on the rightness or wrong ess of some of their actions it just reports that it happened.

Secondly, there are several explicit verses where God says He never commanded the children of Israel to sacrifice children to Him. It is in Deuteronomy and I believe Leviticus listed that it is one of the sins the inhabitants of the promised land were being judged for and that Israel will be judged if they did that.

In Jeremiah God says these sort of things "NEVER CAME INTO HIS MIND".

In conclusion, like you say, the text is not very clear, there are very clear texts where God prohibits child sacrifice so worst case scenario is that Jephthah did something God does not support and at best Jephthah's daughter was devoted to God and Jephthah's lineage was ended.