I cannot use other configuration, like RS422, RS485 because I got no more space on my small PCB.

Make it first, then design it, a sure way to incur problems.

You "cannot use other configuration" because you made the boards before you designed the system.

So, Trash the boards, and go back to designing.

Anyhow, I have the feeling that you are going to do it anyhow and make your own experience, so here we go

1) Is the TX line high impedance in UART mode?

2) Can I connect all the Slaves' RX line together.

3) What about all the Tx line together? My concern is if Slave100th starts transmitting, it will be sinking current from 100 devices, seems pretty scary to me.

4) Has anyone did this before?

1) no, it is "weak pull-up"
2) sure, 3) but can one TX sink 100 "weak pull-ups" that would be derivetive dependent. Also your baud rate will be very small because of the capacitive load
4) I hope not.

So Tp, I will make myself a seer and forsee the following: you are now going to connect all this and you will realize that the max baud rate you can manage is, say 4800 because of the capacitive load. Then you will see a lot of noise problems and to handle that switch to shielded cables which will reduce your baudrate to, say 600. Then you will see that you still have, albeit fewer, noise problem and finally give up and design some boards that can actually handle what you are trying to do.

So, save the trouble and scrap the boards you have.

Erik

Ps
when selecting 485 drivers for your new boards make sure you use "1/4 load" or "1/8 load" chips since you say you have 100 nodes.