using find in bash to operate of filles based on their extension patterns

Unquoted *.txt is bad. See find utility does not output all files when using wildcards.

is it possible to combine these 4 commands into one?

Yes. See Combining multiple find commands into one.

find . \( -name '*.txt' -o -name '*.xml' -o -name '*.pdbqt' \) …

^{(This uses 3 patterns because your 4th command duplicates the 1st one and there is no point in using the same pattern twice.)}

Is it possible alternatively to use find in order to remove ALL files which are NOT belong to the *.dlg extensions?

Yes (and I assume you want regular files, not all files):

find . -type f ! -name '*.dlg'        # -delete

If the above command prints files you really want to delete then run it with # removed.

The ! -name '*.dlg' test is negated -name '*.dlg'. It succeeds if and only if -name '*.dlg' would fail.

You used -delete in the question, so your find apparently supports it. Users with less powerful find see find: -exec rm {} \; vs. -delete - why is the former widely recommended?

Note in your case bare find works like find .. I used find . explicitly because some implementations of find require at least one path in the command line.