The website often has difficulties pricing up a WTP-CW upgrade using Avios correctly (this is often related to a type of fare called Dual Invertory Fares, or DIFs - the website simply doesn't cope well with these and gets the pricing wrong)

Your idea is not only sound, it is the normally recommended method i.e. book online and then phone up to complete the Avios upgrade. If the seats go in the short period of time, you can use the 24 hour cancellation period to cancel the whole booking. You can also phone up and do the whole booking over the phone, but the agent may need some convincing to waive the phone service fee.

All WTP tickets bookable on BA.com will be eligible for upgrade.

For a one way upgrade WTP-CW on that route you will need either 20,000 (on peak days) or 24,000 (on off-peak days), That is not a typo, you really do need fewer on a peak day as the cost to upgrade is the difference in the "straight" redemption fees, and on peak days this difference is less.

Additional surcharges should be in the region of £100, you can confirm an exact amount by doing a dummy booking and comparing the tax breakdown. or e.g. look on ITA Matrix for the TFC breakdown in each case.