You should always put the tongue in same place, but I suppose if you're native and/or fast speaker, you get used to it. The 'th' should not come out as 'd'. For the /θ/ -sound, the tongue doesn't really go firmly between upper- and lower teeth. Instead, the tip of the tongue goes lightly behind the upper teeth. Perhaps just slightly positioned between the teeth. You'll find plenty of pictures online to demonstrate the positioning. If you wish to practice pronunciation, there should be quite a few videos in Youtube that help you get it right.

In fact, we don't generally make 'th' sounds with the tongue between the teeth. Teachers sometimes do this to demonstrate these sounds, but that isn't the usual articulation. Most people say 'th' with the tongue inside the mouth, with the tip briefly touching the back of the upper teeth. The two 'th' sounds ( unvoiced in 'thin' and voiced in 'this') are called dental consonants, because the tip of the tongue is in contact with the teeth. The 'd' sound is different. You pronounce 'd' ( and t) with the tip of the tongue further back, touching the part of the top of the mouth called the alveolar ridge. This is why d and t are called alveolar consonants.

Yes to the first part. "D"? No. don't get you. If you say "D", you say "D". . I think the pull back part is the transition to a vowel, not part of the "th".