I have faced the same problem lately,the culprit in my scene was seepage at the upper tank of radiator sinced i changed that car is fine now,no boiling water in expantion bottle,and yes there are two pipes attached to the expantion bottle's cap,thin rubber pipes,some time they get leak but not much noticeable,you will have to observe keenly for any seepage in any part relating to the cooling systemr,secondly check the radiator cap,its rubber washer should not be torn,try changing the cap with origional japaies one and then observe, is it still boiling water in expantion bottle?,thirdly if these two symptoms dont work out,then get its thermostat valve checked but as you said there is no heating issue then it does not look to me as the valve problem,the problem lies in first two symptoms,or some times more complications occure if the head bolts are not torqued properly but thats again for later concerns,firstly just check the two sypmtoms a told afore.
Always use coolents,better for your engine's cooloing system and a great help to find out the seepage because of its color.
The second problem of your car is related to rpm setting at petrol and gas, if applicable, by some good cng mechanic or if worst, tune up your car including throttle body wash and the TPS (throttle positioning sensor) and its adjustment.
