1) In the case of Carthage it probably was mostly the second. The phoenicians weren't that big in number, but they could relay on Libyans (Natives). However a certain amount of Phoenicians did serve, but only in elite troops (lower chances of risking death,...) For example the Sacred bands,...). They were rich, traders in nature. Not much warriors. Of course citizen miltia could be raised in time of emergancy.
The obvious solution, if you have a lot of money but don't feel like fighting, is hiring someone else.
Romans: see point N° 3
2)Now Iberian mercenaries were used extensively by the Carthagenians. Mostly because of one reason. They had good contacts with them, and you didn't need to travel much outside you're empire borders to get to them. They also had something the native lybians, corsicans, sicilians, sardinians,...didn't have heavier and medium infantry, that could hold lines. Now while lybians armed by the Carthagenians didn't differ much from the greeks, mercenaries using typical lybian armament, well weren't suited to fighting melee and holding lines. Of course they also had acces to greeks and celts at certain times. Though I could imagine (a guess) that greeks might have been more expensive.
If Iberia wasn't occupied by Carthage, or had almost no contact, there probably would still have been mercenaries. Fighting for certain tribes or (petty) chiefs. However they would probably smaller in number. I bet the romans would have used them too afterwards. Though I think that it were mainly the Carthagenians who showed the Romans their use as Mercenaries.
However the Romans did notice Iberian fierceness not only because of Carthage. I took them 2 centuries to finally really control Iberia. Certain great mercenaries might not have been notice by the romans otherwise though.
3)Well from a bit after the start of our EB game, they would rely on their legionaries (Hastati,...) and to a force of the same size composed by ally troops. So about 50% usually weren't really Romans, however you can't call them mercenaries. Of course in time of need Romans could use mercenaries. From about the first century BC the usage of auxilia rose. This was due to the fact that you had to have Roman Citizenship to serve the legions. However because of exapansion, and the amount of borders and conflicts, this wasn't enough. As time went by these levies also became standarized by the end of the first century AD. When the Western Roman empire came to an end foreingers started to make part more and more of the roman Empire. Mostly Germans. There was a decline of population, and the empire had to many borders to protect. However Germans there were plenty, and were happily to serve another commander, if they got something in turn. When we come really close one of the actions the romans took was allowing certain tribes to settle and cultivate in roman territory. As long as they helped protecting the borders. (The first were IIRC the Salian Franks.) Some were easy listeners, others caused more problems. Most did have quite a bit of independance though. Most of these germans didn't have any problems with fighting other german tribes were really loyal and and protecting their territory. They had to do the same thing in Germany, but now they had better land and better (trade,...) relations with the Romans, they just had to serve the roman army.
note this a quick/short and a simplified awnser. If you want a real good idea of it all, you really need a book. Check the EB bilbiography for books about this subject. I can't really recommend an English one on this topic. It shouldn't be to hard to find a good book about the Roman empire and it's soldiers. The other two questions might be harder.
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