Yes, he was a rookie who (a) missed 2 games to injury and (b) fumbled 6 times, which led to a short benching. Not surprising he "lost" 90 carries to others.
Gordon got hurt after just 3 carries in game 13 last season. Through week 12, Farrow had 20 carries and 1 reception, compared to 251 carries and 41 receptions for Gordon. Then, in Gordon's absence in games 13-15, Farrow had 40 carries and 12 receptions before he got hurt himself. The way you wrote this seems to imply that Farrow was a presence while Gordon was healthy, which is not true. Farrow is a JAG who I do not expect to make the Chargers' final roster this year.
As for Oliver, he is probably going to be Gordon's backup, and possibly get some work on passing downs. That is,iIf the Chargers don't sign a veteran RB before the season opens (e.g., Mathews if released by Philly), and assuming that Oliver beats out Barner. But Oliver is another JAG, and he is a JAG coming off an Achilles tear.
I get that you think that Gordon's injury history is concerning, and that is a legit concern, but some of what you posted about Gordon seems off base best case, worst case it seems disingenuous.
You also failed to acknowledge the change to a HC known for successful running offenses as well as the expected OL improvement.