I am not an expert so don't quote me on my answers. About your first question, as far as I know, the energy infrastructure cost is not described in En-ROADS. The only costs are related to energy consumption.
About your second question, I think you can tax all the fossil fuels by the same % (Coal +20 USD/tce = +30%, Oil +15USD/boe = +30%, Gas +1 USD/Mcf = +30%).
I am looking forward to hearing your feedback.
I was able to simulate the difference - and was able to make a carbon tax equal to the cost of the above taxes. It turns out carbon tax is more effective (without simulating infrastructure investments as well) which makes sense.
This did leave me to see that the most effective is an outright ban on new coal, oil, gas infrastructure. Will pursue something else to help with #1.