Yes, breakpoints will still work. Debuggers generally use POKETEXT to write breakpoints, which ignores any write protection on pages. mseal does not affect this use case.

that's an interesting point. i know at the kernel level gdb supports hardware breaks that don't need to add in the trap instruction but I'm not sure if those are available to user-mode processes.

Yes, gdb sets them with ptrace when debugging other user mode processes.