That's right and there are different qualities of chords - many hymns are in minor keys - it would be a very boring world of music otherwise!
That's correct, however the OP has received a song list showing, I presume, only the name of the song and the key to play the song in. Not a lot of help. If he will rely upon I IV V until he can get something a little more detailed he will at least be able to jam a bass line from that.
Now if that is the case - all you have is the key. Here is another thing you can assume.
The first line of the verse will start with the I chord and the IV chord will come in close to the end of the first line.
The second line of the verse will start (continue) with the IV chord and move to the V chord near the end of the second line. The second line will end with the I chord.
I chord starts and finishes the first two lines in the verse. The V is the climax chord and comes into play just before the end of the 2nd line. The IV chord fills up the middle. Where each comes into play - good question and deals with where the melody notes fall. Just get them to give you some sheet music that will go into more detail.
This pattern will repeat itself in the last two lines of the verse. For the lack of better information assume that each verse and the chorus will have this same pattern.
Now that is close, but, close works in horse shoes, grenades and jamming.
I also assume you received just the song name and key, because the old bass only needed that, i.e. he jammed his bass line. I bet they have better sheet music, ask the rhythm guitar.
Another hint - watch the rhythm guitar's fretting hand and change chords when he does.