Dealing with Nasus carries as support

Use your ult on as many people as possible, then focus your effort on the Nasus. You get the CC/Team utility from stunning 3-4 people with your ult which is useful. You then have the opportunity to keep Nasus busy with slows/Knockups.

From here, you have a few item choices.

1) Mikael's Crucible allows save your carry from a Wither allowing your carry to kite and output high damage when they would normally be withered.

2) Iceborn Gauntlet. Try to weave your spells and auto Nasus. This means wait 1.5-2 seconds between each spell activation so that you can proc another of Iceborn's AOE slow. Between your abilities, you should at a minimum be able to keep him pretty perma slowed. This will stop him from getting to your carries.

3) Randuin's Omen. If nasus decides he can't reach your carries and so decides to just pop you. The Active on this allows you to keep Nasus slowed when your abilities go on CD as well as gives you HP/armor so he can't just decide to kill you instead. (Iceborn helps keep you healthy as well, but doesn't give the additional HP this does.)

I think you have a lot of the important things mentioned in your question (especially about warding). I think your role in team fights is much more situationally dependent on your team comp and the enemies team comp.

Sej is one of several supports that can act as hard-engage or as peel, depending on what is needed. If your team comp doesn't have any other champs to engage with, and you think you have caught the enemy when they are not prepared for a team fight, I would leave my ADC to go initiate on one of the enemy carries. However, if you have a Jarvin or other hard-engage specialist on your team, let them initiate and be there to peel in case the fight goes poorly.

Similarly, certain very mobile ADCs (Ezreal, Lucian, Trist to an extent) don't really need the peel as much, and should be able to kite a bruiser without a gap close (like Nasus, applies to some others as well) pretty well on their own, leaving you free to go create havok elsewhere.

In my view, it's not good to waste a big AoE CC to peel from a single enemy, as there is a ton of potential being lost. In the end, you want to neutralize as much risk as possible. If the enemy Nasus is so fed that he alone is a bigger threat than the rest of the enemy team combined, then focus him down, and clean up the rest of the enemies as they scatter afterward.

One final thing to consider is what stage of the game you are in, as this affects what happens after the team fight. Early game team fights tend to result in a dragon for the winner, in which case your jungler is probably a little more valuable, since he will probably have the vs neutral monster stats to do good work on it himself, or he could attempt a steal if the enemy team won the fight. Likewise for mid-game Barons (but you'll probably need a couple teammates to survive to beat him down). If you're looking to get into the enemy base after the fight, you'll need an AD carry to really crush turrets and other structures. Conversely, you might need to defend your waveclear (Ziggs, for example) if the enemy team looks like they are trying to push into your base. Superlate game, if you already have an enemy inhib down, just get the Ace at all costs - Sej and some super-Winions will easily finish off the enemy base when they are on minute long death timers.