i) I believe the official altitude change for 1mb is 27ft, but that is for the International Standard Atmosphere, which does not exist anywhere on Earth. However, 30ft is used by one and all as an easier number with which to work. If you ascend several thousand feet, the pressure is much less than at sea level, so 1mb will be worth more than 30ft, if you can understand that. In other words, the rate of decrease in pressure decreases as altitude increases. If the pressure is half that at sea level, 1mb would be equivalent to 60ft (or 54ft if you are being pedantic).
ii) Yes, you are correct. If the QNH is 33mb either side of 1013.2, which you set climbing through the transition altitude of 3,000ft, your flight level would be either 020 or 040 approximately. The transition level would be adjusted if necessary to the lowest level above 3000ft.