Why would they hit the front line?

The only nuclear weapons Ukraine would need is enough to reliably ensure Moscow and St Petersburg cease existing.

Do you really think the US would trade Washington and New York for a chunk of Mexico?

I think the point about India/ Pakistan is a great counter. At what point would a nuclear Ukraine launch?

India and Pakistan shot each other a bunch over the border. They notably did not launch land invasions even in their disputed region.

What do you think happens once Ukraine takes those two cities out with theoretical nukes in an alternate timeline?

Does it matter? The Russian parliament, most of it's wealthy citizens and all the critical industries would be dead.

Like...that's the whole point of MAD. You might as well ask "well why didn't the US and Soviet Union go to war? Obviously they won't launch..."

The entire point of a nuclear deterrent is it stops people asking those sorts of questions, because ultimately it's a gamble. The only guarantee you get is "don't invade my territory and I won't launch". As soon as you start not doing that, you get to ask if you think it's a 1% chance, or a 2% chance, or or or... You get to find out when your capital and most populous city do or do not explode. And you get to roll those dice over and over again.