Hi there! \n
Do you have SharePoint 2013 Enterprise installed or SharePoint Online E3 subscription plan? These editions supports InfoPath Forms services. Though I didn't recommend InfoPath at all, it's deprecated by Microsoft and using it is the way to deadend. \n
From other hand we will need to look into your forms, complexity and structure. I have great experience of UI modifications in SharePoint adding different controls on standard forms with tabs conditional visibility and so on. \n
Other challenging aspect is offline client, data for synchronization and logic on client. My recommendation is to use WPF client and local DB for storing data with architecture of pulling and pushing data when online connection is estimated via REST or CSOM connectors.
You should also understand that web logic and UI and offline client logic and UI are completely different things and it will be necessary to develop them as separate parts. \n
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Best regards, \n
Andrew Koltyakov