if it applies to summon? well the summon is tied to your main char so if it gives to your main char it should give to summon as well and yes it works on chars with 2 chases, not sure about stacking with other passives but thats another topic, as for combos alone usually they dont do much damage, but there are certain situations when high combo is good

yes it does work with summons, I did a check using a Purple summon and him and it did use the summon chase twice when the conditions were met, so if you were going for a high damage build that also boosts the amount of chases that happen, you c*e summons multiple times per round