1. Not sure what you mean by significant delta-v. An 8km/s vehicle meeting a 6km/s warhead coming from the opposite direction will kill it just as surely as a 10km/s one. If you mean delta-v to go laterally, for that 1km/s is probably more than enough. A vehicle with 1km/s delta-v is, in the worst case scenario, twice as heavy than one with 100m/s delta-v, not 100 times as heavy.
2. 9 minutes is plenty. A ballistic missile takes about 30 min to go from one continent to another. The difference between 4 min and 9 min is not a make or break thing.
3. Everything has a price. If a single GBI booster costs north of $50 mil, then you can afford to spend a few tens of millions to keep fully fueled Starships on the launchpad, 24/7/365. From time to time you'd have to empty them, and move them an give them a thorough inspection. It can be done.
4. (bonus) If things get tense, a Starship could also carry a few hundred MIRVs. China thinks it can sprint forward in the arms race at a certain point. They'll find out it's not going to be that easy.