Fallout – Rebuilding the entire Fallout Shelter

There is no way to force them to use them, they will just use them on their own until their stock you give them when you send them out goes away.

Nothing (that is in the game right now) can cause them to die in a single attack that they would not be able to heal from so no worries there. When they get up to the radiation amount a RadAway would heal then they will use one, then their health is low enough for a Stimpack to fill it up, they will use it.

A good ratio is 2 to 1 from Stimpacks to Radaway. I generally will send out my guys with 20 Stim Packs and 10 Rad Aways though they will often still return with a RadAway or two on them. I will also do 15 and 10 if I am low. Your residents will never need RadAway unless you run low on water so as long as that is ok do not fear sending more with your guys in the wastes.

Neither. Dwellers in the vault take radiation damage where there isn't enough water. As long there is enough water, you don't have to worry about any radiation.

Dwellers in the Wasteland with less than 10 END will take Radiation damage over time. There is no risk of them passing this radiation damage on to other dwellers.

EDIT: As of 1.1, wasteland explorers with END up to 12 take Rad damage, but so little they will use 25 stimpacks and die before they need a single Radaway