The answer to both questions is no. You can only deposit pay and credit hours that you accumulate in a bid period above 80 hours. (The 77-hour bank limit you reference is the amount to which you can withdraw from your bank to fill up.) You can accumulate such pay and credit hours only by picking up additional flying at single pay and credit, e.g., via WS, swap with the pot or pilot to pilot swap.

In your scenario, you have 74 hours of pay and credit, and the GS trip gives you an additional 10:30 double pay, no credit (i.e.,21 hours).

With no bank withdrawal, your GS trip would pay 2 hours pay and credit (to get you to the trigger) and 8:30 double pay, no credit, for a total value of 19 hours.

Bear in mind that you can borrow hours from the bank too, up to 20 hours per bid period normally but only 5 hours in a bid period in which you have flown a GS. In this case, you could borrow 2 hours to bring your pay and credit up to 72 hours. This would cause your entire GS trip to pay double pay, no credit. In essence, the 2 hours you borrowed from the bank would be worth 4 hours.

Keep in mind that banking hours to cash in at a higher rate later does not work out like most think. You are banking hours not pay. If you work one hour now and bank it the rate is the same as if you work that hour next year. Either way you work one hour. Banking hours is best used if you want to build a big block of time off at a later date.

You are correct, it is 80 hours not the ALV.

Exactly. It's not possible to cash out 40 hrs of bank at once unless you PD most of your trips. I don't have the contract in front of me, but I believe you cash up to the ALV+15 or max 82 hours, whichever is less. And if you GS you can't cash out more than 5 hours.

You're better off buying vacation days.

But your rate the following year will be higher. Especially if you are on 1st - 3rd year pay where you get the largest longevity raises. I always banked everything I could during the 1st couple years. If you can bank an hour at 1st year pay of $78 and then cash it in a couple years later for $110 how is that not good deal? Maybe I'm overlooking something. Also, is there a max you can build your bank up to? I thought I read 60 somewhere but I couldn't find it.

That is completely 100% dead wrong.

When you withdraw an hour it is at your current rate, not your rate when it was deposited.

If you make a bank deposit of one hour while you are on probation you can withdraw that same hour the next year at 2nd year pay.

The best time to bank hours is if you are expecting an upgrade from FO to Captain. You can bank those hours at the FO payrate and withdraw them at Captain payrate.
If you play it correctly, up to 60 hours of your time as an FO you could be making Captain pay.

This is not true. As you say, you bank hours and not pay. If you bank an hour as an FO, you lose one hour of pay that month at your FO pay rate. If you later withdraw that hour as a Captain, you add one hour of pay that month at your Captain pay rate.

Close. It's ALV +5 or 82, whichever is less.

The withdrawal limit per bid period is the lesser of ALV+5 or 82.

You could personal drop your entire month and use the bank to still get paid.

Assuming you had a 60 hour balance you could withdraw all of that and borrow 22 hours and you would get paid 82 hours without flying a single minute.

Math in public.... ;)