I think they should, just like building capture, not to overcomplicate things. It would automatically be just more useful for some than for others, particularly since civs that don't build complex ones would be usually more mobile.

I'm not sure every civ should have the ability. But at least the civs that actually captured them historically could have a technology to unlock siege engines. https://tyche.univie.ac.at/index.php/tyche/article/view/5047   This article at least argues for them being Scorpions but the original source is no that clear in any case.

I see. The issue with real time MP (if you consider to add that at some point) is, obviously, that if some (or many) player(s) are engaged in "battle map", for the rest still in "campaign map" time would need to pass "too slowly" (not to lose sync). If some players abuse this, it could get quite boring for the others, so I thought maybe each could have a "focus" bar, depleted while they are in "battle map", regenerated otherwise (that's a simple explanation, this could actually depend more finely on chosen speeds).   Hopefully you can get that from Parts II and III of Thoughts on the Athenians, already sourced from plenty of the most important books and researchers on these issues

It kinda is at the moment actually. It's structured for having one turn where players can play (but armies have movement cap, and everything like training, building is counted in turns), then at the end of a turn, bots do they own moves. Well I haven't got multiplayer as a focus right now so it's missing some piping but nothing is being built that would make it too hard to do.   Not yet, I want to put together all the work I've done past year to make it possible to have some "battalions battles", and maybe go on with a good naval system. I'd like it to have good variety of gameplay, for example currently you have the mini-mission for founding a city under waves of brigands attacks. So an economic battle. Then army battles where armies fight in battalions, and hopefully both sieges and naval battles can be unique too. Another goal is to mostly support creation of campaign ideas. The campaign I'm going to create will probably be very basic story that just try to show all the gameplay features. 

@NiRa Muso MIOZOTO This is the No Violence Mod. All attack abilities are removed. You can't even kill animals, so it seems perfect for what you're looking for.There's also a link to the version uploaded to mod.io, unfortunately, it's outdated. It will need some work to make it compatible with the current version. You could also take the opportunity to change the icon. mod.io link: https://mod.io/g/0ad/m/noviolence-mod