#1 I never found making companion gifts with UT and investigation to be a monetary burden. make sure you make the right conversation choices with the companion you have. This can be hard sometimes.
#2 absolutely not, the droid is the worst companion you can use off-ship.
#3 affection does have some impact on chance of failure but the chance never reaches zero. There is always a chance of failure. This is probably superstitious nonsense but I have found that if I send the same companion on the same type of mission (IE UT metals) in the same grade over and over that companion fails sooner than if I mix it up, i.e. send him on a UT metals then send him on a companion gift, then back to UT metals.
#4 sorry I do not know
Concerning #2 that is not entirely accurate. At Legacy level 12 you can buy modifiers (do not remember exact name or type of item) from repair droid on your ship. Of course that repair droid itself already costs 1 mil, dunno what upgrades do.