Section A: Randuin's a situation item? I have literally built Randuin's on an ADC and haven't felt bad about it. Let's think about what Randuin's does. It gives 500 HP, 70 Armor, a passive that slows atk speed when the enemy hits you, and an active that slows everyone around you by 35% for 2+ seconds.
HP: a useful stat in general, as it means you can take more hits and protects you from true damage. It also makes your resistances more effective
Armor: Every creep and neutral monster deals physical damage, which means armor helps you in tanking Dragon and Baron. Also, Turrets deal physical damage, which means more armor makes turret dives easier. Further, every champion in the game has an auto attack, ergo, armor is useful against every champ, not just those who revolve around auto attacking.
The Passive: This is the part that is used against champs that revolve around auto attacking, however, there's usually at least one on every team, so it never goes entirely to waste.
The active: It slows everyone on the enemy team for at least 2 seconds, usually 3. Therefore, the active is useful for both chasing and kiting. A 35% slow is nothing to sneeze at. Most dashes/blinks have a very long CD, so popping the active forces them to use it and if they already have used it, they're either caught out or unable to continue chasing. This is useful no matter what the enemy team comp.
Of course, the build you take depends on the enemy team comp, obviously if they're full AP, you're going to want to rush MR, however, even against the full AP team, you still want some armor for tanking objectives and turrets. As such, I fail to see how Randuin's is situational in the slightest. It is the most versatile armor item in the game, hands down.
Section B: blade is OK on Rek'Sai, but with the upcoming nerf to her, blade is just a bad idea. She'll definitely need all the AD Hydra provides. Hydra also helps her farm faster and unload greater amounts of burst dmg. Because it's so easy to kite Rek'Sai once she uses her tunnel, she needs to as much upfront dmg she can get. Botrk's active isn't enough to overcome this.
Section C: Powerfarming is never a bad choice. If absolutely nothing else, you're not ganking and giving up free kills to the enemy. Also, even the weakest of laners can hold their own, if some small amount of intelligence is applied. Further, ganking for a weak lane isn't ideal, because they don't have much potential to help in a gank and it's likely they've been poked down, therefore it's more likely for it to turn into a 1v1 or, if you get counterganked, a 2v1.
Section D: If you use tunnels to get in position for a gank, you won't have them for the gank itself, as the CD is pretty long. Also, the tunnels are exceptionally easy spot and get rid of. I have seen games where the Rek'Sai has not ulted at all because her tunnels kept getting destroyed as soon as she left. Therefore, it's more reliable to save her tunnel for the gank itself, rather than try to do something fancy. Also, I feel you drastically overestimate her ganking potential. Outside of her knockup, which she must be on top of them to use, she has no CC and even with the slowing smite, there's no way her tunnel would be back up in time for a second use during a gank. She also doesn't have the damage to solo anyone unless previously fed, therefore she needs the help of her laners to get off a successful gank early, which is hard to get, because of the lack of CC unless the enemy laner is grossly overextened without their escape ability and/or flash. Excluding, of course, cases where your laner has very long range CC, such as a Lux binding.
tl;dr Personally, I feel Rek'Sai is a 3rd tier jungler, however, if you insist on using her over other junglers, these are some of the things you must keep in mind.
