Situation 1 - Dip your fingers in your drink - pretend to sneeze as you flick the liquid on to the offending foot - it will vanish quickly - if it doesn't pour rest of drink on it. If all that fails - a good hard shove back with your elbow - it's your armrest and meant for your arm noth their foot!

Situation 2 - watch the overhead whilst boarding is going on - if someone moves your bag - get up and replace it where it was - if offender complains - tell them yours was there first and contains fragile items.