Need advice on IM attire tomorrow

55 degrees is not at all for riding in a trisuit. I would recommend to err on the side of warmth early in the day. You won't be hammering away and generating much heat internally, but your pace will be generating a lot of cooling breeze. Do you have a long-sleeved base layer that you could throw on?

Good luck for the race! It's awesome how far you've come to get to this point.

I don't know about this part... I wore arm coolers (de soto wings) last weekend on a 4+ hour 100-degree ride and could not wet them for long periods of time. When they were dry my arms did not seem any warmer than the rest of my body? Don't know, but I'd hate for them to have the opposite effect, or intended effect rather - and make you colder?

Do you have arm warmers? Do you think you'll need the arm coolers in 60s and 70s?

I would probably start with arm warmers and take them off later.

I've done a couple cool-morning, warm-afternoon races and I've been good enough in just my tri suit.  I've seen a bunch of people out there at IMCDA and IMLP dressed far warmer than me and I didn't understand why.  A couple of them were in baggy jackets on aero bikes in aero position - aero FAIL!

The earlier suggestion to get a throw-away $10 long sleeve shirt is a great idea  A T-shirt be a lot faster to put on in T1 right over your tri suit than rolling on tight arm warmers over wet skin, and you can drop it in the trash at an aid station when no longer needed.  You spent enough getting to IMWI, don't skimp now!

Get a second one to put in your T2 bag in case the warmer afternoon doesn't develop.  One time at IMC the rain started at Yellow Lake (mile 90 on the bike) and I had a miserable, miserable finsih to the bike and first 13 miles of the run until someone was handing out T-shirts at the run turnaround.

Hopefully NOT on the course and in a waste receptacle, yes??  Otherwise, littering, and/or leaving equipment will get penalized when seen.

Gallon ziplock bag up your tri top in front works much like a vest and is cheap plus disposable.

Tube socks with feet cut off for arm warmers so you can ditch them.

Good suggestion, CHEAP DISPOSABLE ARM WARMER are tube socks from walmart.  Newspaper or ziplock back trick will work great.  We use newspaper for a lot of our fall/winter rides.

Buy some calf-length tube socks at the local drug store and cut the toes off.  Use these as arm warmers (roll them into doughnuts you can slip on your wrists in T1 and roll up your arm on the go).  When it gets warm enough take them off and stuff in your back pocket, or dispose of them at an aid station.

Consider compression calf sleeves as well (under wetsuit).