One suggestion: there is a Bike Touring Americas Facebook group that pretty active and will likely also be able to answer questions you asked here.
I bicycled the Carratera Austral in 2017. However, it was with an organized group who handled most of the logistics so I didn't have to pay as much attention.
However a few thoughts on the questions you asked:
1. I don't know all the places with ATMs. I got both money from an ATM in Coyhaique and food from market there. At minimum I would count on Puerto Montt and Coyhaique. Other places are all a bit smaller but some might have ATMs as well.
2. We took the boat from Villa O'Higgins. Our baggage trucks backtracked to cross the border further north. The sequence of ferrys is very much an experience I wouldn't miss. I believe there was a (new) ferry from Puerto Yungay to Puerto Natales if everything went bad - but you would also miss a big piece. In our case, there ferry was on time, though I've also read journal accounts of people who waited a few days.
3. The Candelario Mancilla Border is interesting. It is ~21km from the dock to the actual Argentina/Chile border. This route is mostly an unimproved road that can be bicycled. After that on the Argentinian side it is 6km of single-track trail. Some bicycled it, but they were much bicyclists than me - and I mostly walked my bike and carried it some of the distance. I had luxury of not having to carry my luggage. I don't know what alternatives exist for transporting baggage - but it is the 6km in Argentina that is most important.
4. Not sure of normal times for the ferry across Lago Despierto, but in our case the ferry left at 5:30pm. That gave us just enough time to bicycle to El Chalten by dark.
5. I remember getting money in El Chalten and Google Maps shows a bank. However, following was my more general experience with money in several months in Argentina:
a. The currency is the peso.
b. There is a limit of how much one can withdraw at once from an ATM, Last year the limit was 2000 pesos, but over the past year, value of the peso has fallen 50% against the USD, so not sure if the limit changed. In addition, pretty much every ATM also charged a 100 peso service fee - so it is costing 5% to get money from ATMs.
c. The ATMs occasionally seem to run out of money. As a foreigner, you can't tell if the reason you get an error is because your card doesn't work, or there is no money.
So it also wouldn't surprise me if you find some accounts where someone came to El Chalten and even with an ATM, wasn't able to withdraw money. I ended up carrying a bit more in Argentina just in case I came across ATMs that didn't have money.
El Chalten itself matches my description of a place that is touristy (i.e. shops that advertise to do laundry by the kilo) and in those sorts of places especially, one can get by with credit cards for larger purchases and I believe the grocery store (with some risk a card doesn't get accepted, so bring different types).
Hopefully that is helpful and I would also suggest visiting the Facebook group since it will have more recent travelers.