Self service for sure supports regular process actions. So I'd be writing this process along the following lines ... The form is presented in self service and you use what validation tools LD provides and the user saves the form. Some calculations are then done within the process to do the additional validation you want. If some corrections need to be done, a suitable action in self service is provided inviting the user to correct the data. Round and round the loop you go until the data is OK. You then provide a Submit action (or some such) for the user to say yes this is ready to be processed. As an FYI this also allows them to collect data in stages before pressing the final go button. I'd also put a response level in place to remind them they haven't finished submitting.
If you or your project managers have had previous discussions with LD about how you wanted to do validation and LD have agreed you'll be able to do it, then I'd certainly be pushing back and asking them to provide whatever is required to do this. But if some assumptions have been made along the way about how LD works, then you may not make much progress. Hence my suggestions about doing things a a particular way because I know it works.
There may be other ways of course and it's perfectly legitimate to ask LD for advice. They have some clever people working there as well as an SI team that can provide additional functionality by bespoke development.