In short do combat modifiers stack?

For example a Knight has charged into combat with a foot soldier but in the previous round his spear broke.

So he's on a +5/-5 because of being on horseback vs. an enemy on foot.

He decides to rearm with his sword as a dual action, this is a -5/+5, so now the fight is even.

However the enemy is a powerful Saxon so the Knight fights defensively which gives him a +10, this makes the final set of modifiers +10 to the Knight (+5 horseback, -5 dual action, +10 defensive) and +0 to the Saxon (-5 on foot, +5 because his enemy is doing two things).

Writing that down that all seems about right and doesn't seem too crunchy a thing to work through in a quick combat.

Is it right though?

Yes

ditto

I'll stack my affirmative to the rest.

Thank you all :)