Please keep in mind you won’t be able to get entirely ‘unlimited’ database storage in Infinityfree or even ifastnet. You do have ‘unmetered’ database storage in both free and iFastNet premium plans, which means there is no hard limit. It is particularly limited by fair usage, meaning how many queries you can execute in your SQL dataase or how much load the database can take, which is higher in the ifastnet premium plans.
You can’t make a request to your API from another origin or domain, but you can make an API request from your own website. This can’t be disabled in free hosting
Of course, you can use it. You can use any FTP client or online file manager in the ifastnet premium plan too. However, if you upgrade, you will have to manage other aspects of your website from iFastNet’s dashboard, not here. Because keep in mind Infinityfree and ifastnet is different, Infinityfree has no premium plan, the ones you see is from ifastnet, which is sponsored