For second question, in mods section i think you can open mod files. You must edit that database.
For first one i will explain with pictures. Select all civs when editing a unit, i don’t know what it does
Add build location for the first unit and look its id. (Its not in this picture.)
Look at Techs, There is civ, age and requiared buildings, thats why you don’t need to change a unit for a civ. Look for effect number (id).
I replaced Catapcrath with Centurion, sorry for typos. I will show how to add secondary unit too.
This is how to copy unit, copy unit then buff stats to make Elite version. I don’t know how to change Language (Description, unit name). Search for Elite Catapracht at techs and look its effect then put Centurion versions instead of Catapracht.
Add build location for the secondary unit and look its id.
Add new Tech and Effect or just go back to make unit (avail) effect where you add centurion instead of catapcrath add new effect to that and select command type (2 for enable/disable objects). Then put unit id (for Legionary its 1). If you want to put at barracks, do that. If you want to make Elite version make a buffed units, add tech add effect and put change unit command to that effect.
For Jannissaries, search for Elite Janissary Tech, look at effect id. Go to effects and find it. There will be Jannissary to Elite Janissary, change Elite Janissary id to 52 (Royal Janissary). Also you can try to copy Elite Janissary texts (Description etc) by giving those id texts to Royal one.
Or put Elite Janissary tech effect to make Royal Jannissaries enabled and give Royal Janissary build location Castles/4. Give them more cost or creation time if you want to (because Royal ones has +5 HP but already created Jannissaries will be Elite one)
Maybe you can find more help there, like texts (name, description etc.) And what does selecting civs on units tab. And also I don’t know how to make/change Tech Tree.