On point 2, re: defense-dominant vs. offense-dominant future technologies—even if technologies are offense-dominant, the original colonists of a solar system are likely to maintain substantial control over settled solar systems, because even if they tend to lose battles over those systems, antimatter or other highly destructive weapons can render the system useless to would-be conquerors.
In general I expect interstellar conflict to look vaguely Cold War-esque in the worse cases, because the weapons are likely to be catastrophically powerful, hard to defend against (e.g. large bodies accelerated to significant fractions of lightspeed), and visible after launch, with time for retaliation (if slower than light).
On point 2, re: defense-dominant vs. offense-dominant future technologies—even if technologies are offense-dominant, the original colonists of a solar system are likely to maintain substantial control over settled solar systems, because even if they tend to lose battles over those systems, antimatter or other highly destructive weapons can render the system useless to would-be conquerors.
In general I expect interstellar conflict to look vaguely Cold War-esque in the worse cases, because the weapons are likely to be catastrophically powerful, hard to defend against (e.g. large bodies accelerated to significant fractions of lightspeed), and visible after launch, with time for retaliation (if slower than light).