This is a partial help, though not a full lock-out solution. The fields for 'Resolving object' can be referenced in calculated fields. You could look at putting a field in the top of the Issue-variant of the form that looks to see if there is a value for resolving object and displays a warning message appropriately as one step.
Secondly, though I'll admit I haven't tried this to guarantee it can be done, you could try setting up a link to the resolving Project with a filtered Typeahead field. I know you can use text-mode filters there, but I've never tried making one filter to only a specific project. Goal wouldn't be to accomplish anything functional, just to make it easier for the user to jump to the right location. If that isn't possible, I know you could string together a URL in a calculated field that made the link as well.
In part the issue is also that people viewing the Project-variant of the form don't know change have been made on the Issue-variant. I don't have a creative idea offhand, but some way of making it more visible or alerting them to look another place for data could help resolve too.
And lastly, depending on volume of the issue and technical ability of the person converting the issues, you could train the people converting to edit the security of the requestors after conversion. By default they have to have at least Contribute with 'Edit Custom Form' permission to submit in the first place, but if you revoke the 'Edit Custom Form' permission after conversion then the form on the Issue becomes read-only to them.
That part is manual, I don't know any automated way to accomplish it, but it will do what you want at least.
