What you'll want to do is:
#1:
"I am asking because Prolific Academic does not allow me to assign participant numbers (every participant automatically gets an ID that looks like "575329e3eb80c4000741a72e")"
Have Prolific Academic forward its unique ID to your Inquisit Web experiment's launch page via a URL query parameter. Set up your web experiment to read that ID from the respective query parameter and use it as the subject ID for your script.
This will enable you to match the data between the two platforms, i.e., you will know which Prolific Academic data and which Inquisit Web data came from the same participant. For details see the "How to interoperate Inquisit Web with online survey packages" in the documentation:
https://www.millisecond.com/support/docs/v5/html/howto/interopsurveys.htm#2: For condition assignment to work, the ID has to be numerical. Because you cannot use the subject ID (i.e., the alphanumerical code generated by Prolific Academic; cf. #1 above), you'll want to switch the /groupassignment method to "groupnumber" in your <expt> elements:
<expt>
/ blocks = [1 = ....
/ subjects = (1 of 4)
/ groupassignment = groupnumber</expt>
...
<expt>
/ blocks = [1 = ....
/ subjects = (4 of 4)
/ groupassignment = groupnumber</expt>
In your web experiment's settings, choose groupnumbers to be generated either randomly (the default) or sequentially. Those options are simple selections from a dropdown menu when you upload your script to the millisecond.com server.
Hope this helps.