High-quality masoretic manuscripts have a hatef-qamets under the dalet.
The masoretic treatise, "Hidaayat Al-Qaari" (§II.L.2.12.6) states explicitly that a hatef-qamets should be pronounced irrespective of the spelling of the name:
Mobile shewa does not combine with a vowel on a letter of the alphabet
with the exception of the four letters אה̇ח̇ע̇. [...]
If one were to say ‘Is not shewa combined with qameṣ under the dalet
of מָרְדֳּכַי [...]?’, the response to him would be as follows. This
does not contradict what I stated. This is because the people
responsible for this matter have agreed on the rule of combining shewa
and a vowel only under the four letters. It is said, however, that
some scribes wanted to remove uncertainty from places that may lead to
error and have combined a vowel with shewa in this way, because they
thought that people would err in the reading of מָרְדֳּכַי. When some
people saw shewa without qameṣ in מָרְדְּכַי, they read it as pataḥ
[i.e. as a sheva na]. If they saw qameṣ alone, they were at risk of
giving the qameṣ its full length. So, the scribes decided to combine
them so that this degree of uncertainty be removed.