VR4300..( uPD30200 )

Regarding the VR4300 external interrupt pin (INT1 pin), the User's Manual describes "level trigger", but with which internal signal (clock) are signals sensed?

With Sclock.

Is the INT1 signal latched internally? Or is the external signal connected as is to the Status register?
(Even in the case of internal clock synchronization)

Latching is done at the input stage.
Latched items are input in the Cause register.

When the INT1 signal is enabled only for one external bus clock, is there a possibility that the external interrupt exception will occur in the CPU?
(The INT1 pulse width is 20ns based on the following usage conditions: Input clock:50 MHz, internal clock: 100MHz, bus clock: 50MHz)

If the INT1 signal is input even for only one clock when all the conditions are met, i.e. the IM's corresponding bit = 1, IE = 1, and EXL/ERL = 0 clear, an interrupt exception occurs. (IM, IE and EXL/ERL are in the Status register.)

I believe that when a maskable interrupt (IP0 to IP7) occurs, the CPU (hardware) autonomously sets bit 0 (IE) of the Status register to 0 the instant the CPU detects the interrupt (rising edge of the clock): is this assumption correct?

No.
When a maskable interrupt occurs, the CPU sets the bits corresponding to IP[7:0] of the Cause register.
Since the IE bit of the Status register you point out is the interrupt enable bit, it should be set by software.
In the case of IE = 0, all the interrupts are disabled, and if IE = 1, all the interrupts are enabled.

How can I prevent the timer interrupt (IP7) from occurring?

To mask only the timer interrupt, set IM7 of the Status register to "0", and set the IE bit to "1".

The description of the timer interrupt includes the following:
How is the Cause register cleared? (Can only IP7 be cleared?)

Clear IP7 of the Cause register as follows.
Example:

          mfc0 r2, r13
          li   r3, 0xffff7fff
          and  r3, r3, r2
          mtc0 r3, r13

I have a question about the SYSAD bus of the VR4300 and VR4310 Series.
If EValid#, which is a data valid signal from an external agent, is always low, are there cases when the bus is stopped on the CPU side?

Make EValid# active only if passing valid data to the processor when the bus mastership lies with the external agent (PMaster# = H).
In cases other than the above, the operation cannot be guaranteed when EValid# = L is set.

Do any hazards occur when accessing general-purpose registers other than the CP0 register?

No, no hazards occur when accessing any general-purpose registers.