Nope, sorry. You're wrong on both counts.
Item #1. There would be no net difference to profitability from selling McT and Sancho, even if they were on the same wages. I thought it was clear enough just going over things high level, but if the math helps I can show that too.
McT 24/25: £3.1M salary, zero amortization fee.
McT 25/26 (assuming option exercised): £3.1M salary
McT if sold: zero salary, £25M fee
Net difference: improves our profitability by £31.2M over two years compared to keeping him. Frees up the full amount this year
Hypothetical player with McT's wages but £70M transfer fee - 24/25: £3.1M salary, £14.4M amortization (72M fee divided by five year contract)
25/26: £3.1M salary, £14.4M amortization
If sold: zero salary, £25M fee. Amortization gets pulled forward, meaning we take a £28.8M amortization hit this year, which reduces our spending power. Importantly though, the amortization fee is a sunk cost, so we still get all the cash
Net difference: improves our profitability by 3.1 + 3.1 + 25 = £31.2M
The money would be the same for both players. The previous transfer fee paid is irrelevant, and the concept of "academy players are pure profit" is bogus except on short-time horizons. Long-term it makes no difference.
Item #2. Yes, the £28M in remaining amortization for Sancho's transfer fee from BVB is a sunk cost. It will be on our books no matter what we do with Sancho: play him, extend his contract, loan him, sell him, etc. That's amortization remaining from the transfer 3 years ago and there is nothing we can do about it.