Some answers in red
I read in the manual addendum that Engineers speed construction of bases, production facilities and bridges. Does anybody know by how much? Do two Engineer units double the benefit? Do I need to give the Engineer unit specific orders, or does the benefit come simply by positioning the unit in the same hex?Exactly how much per engineer ? Probably only the devs know the exact breakdown.
BUT a full stack of 7 (full str) engineers can circa triple build speed.(If your short on resources ,keep in mind circa triple build speed equals circa triple resource use by that project as it builds)
An engineer will assist building project by just being at least as close as an adjacent hex.There is a construction order,but its use is unnecessary.
Do Engineers support combat in any way, such as against fortifications or bases? What about crossing rivers?
No,they dont assist in combat in any way except to shoot .
They can be used for bridging though