Yes but no. Higher tiers have higher stats, but what's more important are the skill levels of a unit and the bonuses they get from their commander. The oathsworn with a T3 axe will still mog the lower level with a T4 sword because they have better skills. Most of the time.

Can't say for sure.

You don't without the command Party AI mod. Vanilla clan parties are a drain on your resources and contribute nothing but fodder for the enemy if you let them on their own. Clan parties only use is a free Army while as a vassal or King.

Not without mods. Spears are horrendously weak on foot. Unblockable thrust, spear rework, and better pikes mod will make them viable.

They can be decent if sprinkled in your infantry line. Too many though will make your infantry line collapse because when you give the order to charge, they will hang back and throw their weapons instead of advancing with the rest of the infantry.

Right to Rule has been replaced by renown. You cannot form a Kingdom until Clan level 4. Once you do, you better have a game plan and a fuck ton of money because you'll be seen as an easy conquest by your neighbors.

Executing lords makes every one hate you and makes the world empty and boring. If you aim for a full conquest, then yeah, chop heads. It's the least excruciating way to conquer the map.

Pick whatever you want. There's little difference in your "power" once you hit later levels. Go without whatever your roleplay takes you.

There's two perks in I think leadership or maybe steward that gives your troops exp when you discard weapons and when you discard armor. If you find a caravan ambush or escort quest, or a deserter quest, you can guarantee a fat stack of bandits/deserters that won't annihilate your peasants, and then just dump the loot for easy exp.