2. What you're describing is not assimilation but yod coalescence. The yod is the sound /j/ When it follows the sound /d/ , the two merge ( or coalesce) /d/ + /j/ = /ʤ/. So for example "Would you" could be pronounced as /wʊd juː/ but in connected speech is liable to become /wʊʤuː/
Similarly /t/ + /j/ become /ʧ/. So "Got you!" could be /gɒt juː/ but in connected speech is liable to become /gɒʧuː/
If you look at your examples, where the word has an alternative pronunciation in careful speech involving /dj/ or /tj/ as an adjacent phonemes, - eg/
prɒsiːdjʊə/ then in rapid connected speech
one of the potential changes will be due to yod coalescence. Plus of course other changes such as vowel weakening - so you'd end up with something like - /
prəsiːʤə/
It may also occur with /z/ + /j/, As in "As you're..." - with could be pronounced (carefully)/æz jɔː/ but in connected speech might well become /æʒɔː/
But ist has nothing has nothing to do with words like "pleasure", which could never be pronounced */plezjə/ It doesn't exist. The final consonant in "pleasure" is always /ʒ/:
/pleʒə/
So : where there is a potential pronunciation that involves the phoneme sequence /dj/ /tj/ or /zj/, then yod coalescence may occur so that the sounds merge and become /ʤ/, /ʧ/ or /ʒ/. It's as simple as that. Syllabification and other vowels in the word have nothing to do with it