Coolant Top Up and Water Seep by Pedal. Help Please

Hi Ladies and Gents

Hope all is well

So upon starting my car on Sunday (E90 320i) i got a low coolant sign for the 1st time. I didn't drive it after that as I was scared for overheating while driving , and didn't want to fill tap water.

So i have bought the BMW Coolant and I just want to top up, as I do not see any leaks on the floor below the engine, but recently water have been seeping up from my accelerator pedal and 3 weeks ago my car on the drivers side back and front had a pool of water (about 4 cm to 5cm in height standing back to front on drivers side carpet flooring only, passenger side dry dry dry) and no sign of entry anyway else , but by the accelerator pedal (I park at a gradient car facing down and water builds up by the pedal) , but I suspect that from the rain, coz I noticed it after I drive in the rain , but I do not know coz I suspect if it was leaking then it should leak in the front , and not into my car by the pedal . Please voice your ideas on this.

I looked at the level and it is low , so here is my question

Do i put 50/50 water / coolant , I phoned BMW and they said just put the coolant like that , but the bottle says that mix 1 lt coolant with 1 part water.

Do i start the car and leave on idle and then put the coolant mixture , or I just put it in when it's off(I didn't start the car for 4 days now)

Do i mix the coolant and water together 1st , or can I put it separately 1st the coolant then the water ?

Please help me , I have booked the car in for a oil service next week , so they will also do a pressure test to see for any leaks.

Thank you and Regards
Your assistance will be highly appreciated