OIRQGE on all ZX Spectrum models can disable only ula port #FE but all other port is always enable.

ULA work correct, only PAL10H8 (used as adress decoder) contain bad firmware. Is possible replace this chip with modern GAL16V8 with new bug-fixed firmware.

Info is here:
http://velesoft.speccy.cz/zx/umbrella/umbrella.htm

READING of port #7FFD is dangerous only on ZX128 and ZX128+2(grey) models. After IN #7FFD old PAL chip generate clk signal for write data bus to paging port (write actual state of data bus - videoram data of #FF)
Umbrella chip ignore reading of port #7FFD and fix problem with rain (snow) effect on screen if register I is set to slow adress space.

Exist next way for remove RD #7FFD bug = after read port #7FFD must external device on data bus send last writed value to paging port.