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Under normal circumstances, it will not be blocked.
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No, don't worry.
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It shouldn't be blocked.
Do not install a shifter, even if there is a gap in the back.
Use as big as you need, the good toilet has three pit openings, and the bad one has two.
It doesn't have to be that big, I guess you're talking about the size from the back wall, but it doesn't matter, there are some very avant-garde designs with such products, you just need to provide the size range and then supervise the installation quality of the bottom flange to basically ensure that there will be no leakage.
You can disassemble the shifter, rearrange the water pipe around it with cement, repair the shortage of water pipes, and re-fix the toilet bowl after the cement hardens or the re-applied tiles can be used.
The shifter can move up to 10 centimeters, but to dig the ground, I don't know if you don't talk about the hole spacing enough, how far away from the wall is your sewer pipe.