So, Scenario 1:
Firstly, A decent ADC would not push lane without the support. Period. Secondly, ADC's Hardly ever engage. That's the tank/support's job. Just because the support doesnt back doesnt mean they will use a bunch of spells and all of their mana. You seem to have forgotten that support's are on this post for a reason. We know what we are doing and we dont get enough credit from the carries for it. Thirdly, the support hardly ever does nothing. they are always the one shielding the carry when they can and they see damage incoming and/or disengage the enemy.
Scenario 2:
Firstly, Your second sentence describes the exact same scenario from the end of the first scenario. Please tell me if i'm wrong about that. Secondly, a normal support would not be able to get 900 gold in that same amount of time without upgrading a
into a
(giving 5 gold each time the ADC gets a CS and 4 every time he doesnt). The normal route is to back at ~ 1200 g for a
+
then swap your trinket for a
. You get more gold income, and you can get counter ward income. Thirdly, why would the friendly tower lose 1/4th of its HP? If the support stays and prevents the carry from doing damage to the tower, then the friendly turret would have ~ 1/8th of its HP missing (normally considering the armor bonus before 15 minutes). Then the ADC can come back into lane, get some CS, and shove lane back to their turret, lowering ~1/2 of it's HP. Who's to say that their ADC lost that much (if any) HP at all?
Keep in mind this is all without the assumption of any ganks, kills, etc.
Please tell me your thoughts on why you think you're right. I'll be sure to disclaim them as falsitude and give you our (support main's) perspective on how laning should go. Especially the early phase.