For durative verbs this is quite easy for me to grasp:

鳩が見る: I will see a dove
鳩が見ている: I am seeing a dove
鳩が見た: I saw a dove
鳩が見ていた: I had been seeing a dove

But for perfective verbs — it is quite hard for me:
杪冬の前に雪が溶ける: Before the end of winter the snow will melt
杪冬の前に雪が溶けている: Before the end of winter the snow will already be molten
杪冬の前に雪が溶けた: Before the end of winter the snow melted
杪冬の前に雪が溶けていた: Before the end of winter the snow had already been molten

In both of the last cases before winter began the snow is already in a state of being molten as a result of the melting being complete. So I often make mistakes differentiating the last two cases and treating them as the same even though I consciously know the ～た emphasizes the action being completed and the ～ていた emphasizes the state change being already completed. Maybe someone can help?

As the title says: Why is e.g. 靴を脱いだとしたら、家に入り下さい considered unnatural and 靴を脱ぎ終えたら、家に入り下さい is the more natural way of communicating "Please enter the home after you have finished taking off your shoes"

If all V[過去形]としたら are unnatural usage — what is the deeper grammtical reason for it being unnatural?