If there were no Star Trek

Not as much, no. Often there is some resentment against actors who replace a favorite in a series, even though 'Paris" replaced the 'role' on the team, not the 'role' portrayed by Landau. IMO, he was not as good as Landau, but did enjoy a better "replacement reception" due to his time as Spock.

I've read that Landau was originally considered for the role of Spock, but didn't want to commit to a series, and recommended Nimoy. Landau STILL didn't want to commit to a series at the time of MI, which is why he was always credited with "Special Appearance by", instead of being listed in the credit roll with the rest of the cast. He lasted three years anyway.

Leonard's versatility made him memorable for me since I watched this show before Star Trek when I was a kid.

If you read Nimoy's book I Am Spock, he says quite plainly that the show was fun at first, but he quickly grew bored of the role's repetitiveness, and opted out as soon as possible.

From there he moved on to In Search Of... (although in spite of lasting several years, this show is not mentioned even once in his book).

No. He'd be remembered for his role in Invasion of the Body Snatchers.