One of the issues would be logistical getting sufficient backup equipment to Polish airfields would need a large number of transport flights which could panic the Russians, in addition the accommodation and infrastructure available at these airfields is poor, (think east bloc).I do not understand why RAF and others are flying missions from UK to the borders with Ukraine: gives them less time on operational station.
Why not temporarily station some crews in Poland. I know they may have to return to base for more major maintenance, but airlines manage to night-stop aircraft all over the place.
Currently there is a massive training up situation happening in the UK and other NATO countries as most squadrons rarely have a full set of Pilots who have trained for every scenario, so they will be trying to get the younger in service pilots up to speed. If you have been watching Flightradar24 over the last 3 weeks there are many more fast jet and cargo aircraft, A400m C130 etc flights in the UK airspace.
As regards Military Transponders on or off, in the UK most training flights will have Transponder ON thus the aircraft will show up on Flightradar24 or the other plane tracking site, once a flight is tactical then transponder goes off and there are specific unmarked airspace and heights that in particular fast jets will transit through, these are designed to separate Civil from Military aircraft. In addition Military aircraft have a vastly updated and secure IFF (Identify Friend or Foe) system which will tell controllers who and what the Aircraft is.
Hope that helps.