In Finland you can use cards for pretty much everything. Cabs included. Amex included. (I think there are laws against discriminating on cards).

Check-in online and PYOB/mobile boarding pass. Avoids the trek across to the farthest corner for check-in in T3.

Fast track varies, can be quick, can be slow. The dragons can be a pain, so make sure you have the appopriate cards ready. I think BA Silver on BA flight code get access even in ET.

Personally I like T3 galleries as there is also the bag storage available for everyone. Early morning grab the bacon rolls! Mid-day the terminal and lounges are quiet.

If you are on a BA plane its a bus (gate 24, bit of a trek), if you are on BA code-share (eg BA6080 type number) then its a gate and a Finnair jet. I would sacrifice the gate to fly on a BA jet (better sandwhich and free beer).

The viaLounge is perfectly pleasant (I quite like it - particularly the recliners in the upper level are so comfortable!). If you want something different and dont mind spending some cash (not OWE) then pay to go the Spa - its a bizarre experience in an airport. The lounge drinks are average (better just for a bloody mary, soft drink or beer), and food minimal(just snacks) so better to eat in the cafe just outside. If transferring, the wine bar on the schengen side can be good.

Generally a very efficient airport.