It is still 100 days for RM100

the subsequent RM50 and RM10 reload will not adding the validity period because validity period will start from the day you topped up

When you topped up RM50, you validity period is 50 days starting from 20Feb, the system will choose the longest validity period which is the one that end at 29 May.

If you topped up RM50 on 30 April for example, Your validity period now will be 30 April + 50 days. Since 30 April + 50 days has succeeded 29 May, your credit will be valid until 30 April + 50 days

if you want longer validity, yes it is.
People reload because they need'em (SMS/Call/Roaming/Internet) , not to get longer validity period.

same