Ok, but your torpedoes still have to travel to their target. So you have time for defensive measures there. I think you're relying a bit to much on the "before they have a chance to respond". What does that even mean? What does response time look like? How fast can your ship launch a volley of torpedoes? There's just too many variables to make a precise conclusion. I think we'd have to come up with some actual criteria so we could come up with actual conclusions.
Aside from the actual logistics, my first response would be to send up satellites to gather as much data on the attackers as I could. What effects does their FTL drives produce. Then I would try affecting their drives through various means. The goal being to knock at least one of their ships out of commission so I could reverse engineer their drive. I'd try and set up a trap to lure these FTL ships in to a small an area as possible. Space is big so a good chance to have an impact would be to bring them in close.
It would be a fascinating exercise to contemplate.
This is an interesting idea. What would a "border" in space even look like? In the vast distances between star systems, how are you going to detect ships crossing an imaginary plane? How are you going to secure and monitor such a thing? Thinking about it, "borders" have to be inherently based on star system spheres of influence. so your "border" would be more like a bubble rather than an arbitrary plane drawn through space.
I think that having warp drive as opposed to slower than light drive would give the Earth government a vast advantage.
If Earth knows when and where Romulans have been attacking, it should be fairly easy to predict when and where Romulans will strike next, and have large battle fleets warp in and wait for the Romulan attackers to arrive on the predicted times.
If Earth knows where the Romulans have attacked and knows where Romulan planets are not, that should give them a good idea where Romulan planets are. So Earth could send many FTL scout ships and probes to many stars in regions where the Romulans might have bases and colonies.
If a ship or probe sends a subspace or courier drone report that there are no Romulans in a system and it is going on to the next system in its list, fine, Earth knows it is not a Romulan system. If a ship or probe sends a subspace or courier drone report or returns with a report that there are Romulans in a specific system, fine, Earth knows it is a Romulan system. If a ship or probe sends no subspace or courier drone report after a scheduled visit to a system on its list, then it was probably destroyed by Romulans in that system, and fine, Earth knows it is a Romulan system.
If the Romulans are limited to communication by radio waves or slower than light ships, it may take years, decades, or centuries for a Romulan system that destroys an earth scout or probe to report it to the nearest Romulan outposts and/or to Romulus. And it may take hours, days, weeks, or months for Earth to learn that a scout or probe has been destroyed there and add that system to its list of targets.
So Earth would send large battle fleets to each and every Romulan outpost, settlement, or base that was reported by scouts and probes, a fleet large enough to handle the reported Romulan armaments with ease. The battle fleets would zoom to each target system, take battle position at each base, settlement, or outpost, and blast it, smashing the defense system and then destroying all Romulan buildings and structures on every planet and asteroid in that system, before moving on to the next target.
When all the reported Romulan bases, outposts, and settlements had been destroyed, the fleets would unite and one by one hit every system where their scouts or probes had been destroyed and hit those systems with everything they had. With slower than light travel and light speed communications, Romulan colonies and Romulus itself would only learn that Earth was striking back against the Romulans when an Earth fleet arrived with overwhelming force. Remember that Earth ships would arrive before any reflected radar beams of them or any radio warnings arrived.
So if Earth had faster than light travel and maybe communications during the Roman War and the Romulans did not, the only two ways the war should have ended would be 1) total extermination of all Romulans or 2) total conquest of the Romulan Empire.
Thus I find it almost totally impossible to believe that the Romulans didn't have some form of faster than light drive during the Romulan War.