I really dislike
Leonard anyway you pronounce it. I think it's the -nard sound that is just really unattractive to me.
Lennart is somewhat better, especially if you pronounce the t at the end as a t.
I don't think there would be any spelling or pronunciation issues, at least not in the U.S.
I don't think it's common anymore at all.
Sure, why not? But
Lenny seems more natural.