Whats the end goal here, engine & transmission removal? If so, there's probably no need to remove that lever. You'll just have to be extra careful it doesn't catch on anything in the removal process.
But when it's time for the lever to come off, soaking in penetrating oil (PB Blaster, etc) and possibly heat will be needed to remove that retaining nut. Heat will damage the seal, so be advised.
If you don't ever need to remove the lever, it looks seized on there enough that a locking nut isn't really going to add any additional security.
From the looks of it, you should be soaking the exhaust fasteners already... I'd expect some similar challenges there too.
I wish you luck. I grew up in the rust belt and every undercarriage looked like that or worse... definitely have some experience with rusty and broken hardware. not all that fun and I don't miss it a bit.