Silicone is a flexible joint between two planes not meant as waterproofing. Your leak is likely the result of a breach in the waterproofing system (whatever was or wasn't used) under the tile. The silicone may be slowing down the water flow right now, but not for long.

He’s a hack, and created a leaking shower because he didn’t know what he is doing. It’s not built correctly. Period.

Get as much of the $1.98 back that you can, and hire someone that knows what they are doing to redo it completely. It won’t be $1.98.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

Not just retile, but probably demo and start over. The tile is a “decorative cover” placed over a waterproof shower. Your shower is not waterproof.

Was this worked permitted and inspected?

Watch this ten minute video on what should have been done before the tile was laid. If it was done correctly you should never had a leak.

Note: Just using KBRS product install as an example and I am not endorsing them. Most manufacturers of shower kits use the same basic install procedures.

https://www.youtube.com/watch?time_continue=293&v=jO01rUUObkw

"So no other solution than to retile and install a new curb? Ugh.... and the glass needs to uninstalled as well. What a headache. Can it be a partial install of just that section?
Any other options?"

Sam:

You may be able to get a custom solid surface pan fabricated and installed for less work and hassle than a tear-out, but no guarantee.

Looks to me, like you may have a place for water to get in on the upper left hand side of the curb in your picture.

Water coming in anywhere is a non-issue if the bottom of the shower, under the tile, is not waterproof. It is a tear-out, the professionals are telling him.

Tile and grout are not a waterproof system. Grout is permeable. There must be a waterproof layer under the tile layer for a tile shower to be waterproof.

Sam, the water may not flowing be over the threshold. It may be seeping through the floor.

This is not a partial tear down.

There is a lot of info online.

Can you use google?

Yes, Sam, thats the area, I was referring to. I have to agree, this is probably a total tear out, but, please do not hire the same contractor. Do your research well, and find someone that actually knows what they are doing. Its better to pay some extra money to get it done correctly, and it will last for years, than to pay less, and end up having to redo in a yr or less. If you could be there when it is redone, that would even be better. Watch those videos, and read all about proper installation, and waterproofing, and materials to use, and then be a guard dog, and watch your contractor like a hawk, and make sure it is all done 100 percent correctly. If you see even one little thing, being done differently, or not right, question it immediately and make sure it is redone. Your contractor may not like working for you, but, thats not really important. What is important is the job done right.

Jack hammer time.

I am willing to bet that if you ask the guy if he did a Shower Pan Flood Test as required by code.

He would ask.. "What is that??"