1) I don/t think the issue is with B4X. You can write performant code with B4X. The issue is Android. If you really want to build an Antivirus software for B4A, good luck navigating all the restrictions of Android (which become more and more with each version) that may impede or make it difficult to write such a project. Some restrictions, such as related to acceptance of the app by the play store, can be avoided through side loading.
2) Can be accomplished either via Kiosk (device admin style) mode or abusing accessibility (at which point it cannot be uploaded to the play store).