He needs to enable TLS in the game  options  

The reason is scenarios are new or updated maps of each version, and there were no new and updated maps for R28

Night/Day and terrain are not exposed to the simulation so no. Rain are particles, so you can spawn and unspawn them.

Do you know if you can change map settings (like night/day, rain, etc.) during the game?

Honestly reading through these posts, I would recommend if any of you have total war rome 2, to check out the Divide et Impera mod. Granted TW is a in a completely different ballpark in terms of genre, but the DeI devs have managed to make every unit type unique across the various cultures, sub-cultures, and factions. Take for example some of the celtic and gallic factions, they managed to balance 5 different playable factions (scordisci, boii, nervii, galatia, and arverni) to each have different strengths and weaknesses in terms of roster balance.   Managed to steal kindly borrow this chart from the DeI devs. Their reasoning for it was they wanted all the playable celtic/gallic factions to be equal in terms of overall strength, but each are specific in what they're strong/weak in, all represented in the 1-5 relative scale scoring, with 1 being the weakest, and 5 being the strongest in terms of unit strength. For example, the Scordisci have some of the strongest hoplite/phalanx/shieldwall units (weird TW specific term, just refers to units that have access to a phalanx like formation), mostly reasoned because they were in contact (and probably conflict) with the illyrians, dacians/getae, thracians, and greeks. In contrast, the boii, who live in/close to the germanic peoples, do not, but they have far better archers.   Granted all of this is purely stat balancing, but that encompasses stuff like weapon stats, shield and/or armour stats, entity stats (aka the actual soldier stats like movement speed, melee attack chance, melee block chance, etc) and other things.