I'm not sure if there are any FP 1 units with Extended Range, but if I had one, I'd probably say round down (like Opportunity Fire w/ Ops Complete marker).
You can use both AM & ER. As 6.1 is written, you would apply Extended Range first, then AM second.
I'll have to double-check with Mark about negative Firepower totals. I'll reply again afterward.
i think negative Firepower ("-1") is legal: it means that Attack die roll will have a modifier of "-1" (besides the other modifiers)...are you agree?