Can Force Adept (Diala 1xp card) be used multiple times per attack or ability test?

Also, are there any restrictions on how many times you can reroll the same die during an ability test (assuming multiple valid sources of rerolls)?

Every ability can only be used once per timing instance. And Force Adept says it can only reroll 1 die.

FFG has ruled that you can reroll attribute dice any number of times if you get more than one reroll from separate sources. This is probably because the RRG explicitly only sets the a-die-can-only-be-rerolled-once restriction on attacks. You may choose to house-rule it to cover all rolls, including attribute tests.