The rating of the MCB is to protect the cables that are connected. So for example a 2.5 mm² cable has a rating of 24 A**, then the MCB should be not be 25 A, but 20 A.

1. Yes, must change to RCBO, or add RCCB with the C32. If wire is 4 mm² can maintain C32, but I would strongly suggest C25. (Maximum 5700 W water heater with C25) If the wire is 2.5 mm², then technically you already cannot use C20 as Suruhanjaya Tenaga say cannot use 2.5 mm² wire for water heater. Realistically the water heater won't be at maximum heat, so you can get away with a C20 RCBO. (My own instant water heater with rain shower and warm enough for me draws ~16 A measured)

2. Again mainly depends on the wire size. You should also look at the rating of the appliance. For 3500 W, then you would have about 16 A at full cooking power. So, going down to a C20 would be safer no matter the wire size.

3. It does sound extremely oversized. Most hoods are less than 1000 W, so a C16/C20 would be fine.

My own unit:
Lighting 1.5 mm² MCB C10
13 A socket 2.5 mm² MCB C16
A/cond 2.5 mm² MCB C16
Water heater 4 mm² MCB C20+RCCB 10 mA -or- RCBO C20 10 mA
Specialty appliance 4 mm² C25 (Oven for example) Edit I have a row of 3 x 13 A sockets at my kitchen counter. I can foresee myself using an air fryer and an induction cooker while boiling water at the same time. Individually they are not more than 1500 W each, but collectively to the looped sockets it is more than C20. Hence the wire feeding these looped sockets are 4 mm²
Note that ST says air conds are to use 4 mm² wires, but I think that is absolutely overkill as a 2 HP a/cond is at maximum 2000 W (~9A). So I don't follow ST rules to the letter. I'm okay to match the MCB to the wire to the power draw of the appliance.

** This rating depends mainly on the heat that is produced when the wire is flowing the rated amperes. This 24 A is the rating at best case, when the heat can dissipate from the cable. Make that situation worse, by burying that cable in the wall, run 3 phase together, run lots of wires together, etc, and that rating will reduce.

Technically cannot. ST requires a RCCB 30 mA for 13 A sockets. Your DB should have 2 RCCBs, the 30 mA one feeding all the sockets and appliances, and a 100 mA one feeding all the lighting.
-BUT- you could have one RCCB 30 mA feeding everything, as ST specifications say MAXIMUM rating of the RCCB. So using a 30 mA for lighting does not exceed the 100 mA specification.
Actually, ST recommends using 3 x RCCB, one for each phase-neutral pair for residential applications. This is because you may be 3 phase, but you are actually 3 single phase for residential.

Technically cannot. ST requires a RCCB 30 mA for 13 A sockets. Your DB should have 2 RCCBs, the 30 mA one feeding all the sockets and appliances, and a 100 mA one feeding all the lighting.
-BUT- you could have one RCCB 30 mA feeding everything, as ST specifications say MAXIMUM rating of the RCCB. So using a 30 mA for lighting does not exceed the 100 mA specification.
Actually, ST recommends using 3 x RCCB, one for each phase-neutral pair for residential applications. This is because you may be 3 phase, but you are actually 3 single phase for residential.

As for ST guideline:
MCB 16A for 1x 13A socket
MCB 20A for 2x 13A socket
MCB 32A for 4x 13A socket
MCB 32A for 6x 13A socket in ring configuration.

Thank you. Got to plan for my house reno next year. At least I learn some knowledge so not be easily deceive by contractor.
Thank you. That helps in my understanding

The rating of the MCB is to protect the cables that are connected. So for example a 2.5 mm² cable has a rating of 24 A**, then the MCB should be not be 25 A, but 20 A.

1. Yes, must change to RCBO, or add RCCB with the C32. If wire is 4 mm² can maintain C32, but I would strongly suggest C25. (Maximum 5700 W water heater with C25) If the wire is 2.5 mm², then technically you already cannot use C20 as Suruhanjaya Tenaga say cannot use 2.5 mm² wire for water heater. Realistically the water heater won't be at maximum heat, so you can get away with a C20 RCBO. (My own instant water heater with rain shower and warm enough for me draws ~16 A measured)

2. Again mainly depends on the wire size. You should also look at the rating of the appliance. For 3500 W, then you would have about 16 A at full cooking power. So, going down to a C20 would be safer no matter the wire size.

3. It does sound extremely oversized. Most hoods are less than 1000 W, so a C16/C20 would be fine.

My own unit:
Lighting 1.5 mm² MCB C10
13 A socket 2.5 mm² MCB C16
A/cond 2.5 mm² MCB C16
Water heater 4 mm² MCB C20+RCCB 10 mA -or- RCBO C20 10 mA
Specialty appliance 4 mm² C25 (Oven for example) Edit I have a row of 3 x 13 A sockets at my kitchen counter. I can foresee myself using an air fryer and an induction cooker while boiling water at the same time. Individually they are not more than 1500 W each, but collectively to the looped sockets it is more than C20. Hence the wire feeding these looped sockets are 4 mm²
Note that ST says air conds are to use 4 mm² wires, but I think that is absolutely overkill as a 2 HP a/cond is at maximum 2000 W (~9A). So I don't follow ST rules to the letter. I'm okay to match the MCB to the wire to the power draw of the appliance.

** This rating depends mainly on the heat that is produced when the wire is flowing the rated amperes. This 24 A is the rating at best case, when the heat can dissipate from the cable. Make that situation worse, by burying that cable in the wall, run 3 phase together, run lots of wires together, etc, and that rating will reduce.