Just watch RETURNING TO JEDI; you actually have Harrison Ford doing a commentary during both the Sarlaac pit scene and the end battle at Endor saying that Lando was supposed to die at either location (no doubt the Sarlaac death was an earlier draft).

One thing that always bugged me even as a kid (after the 10th time watching JEDI naturally, when you get over the WOW! effect and start to really watch the movie) was: how is it that the reactor just exploded and it takes the Death Star like 10 minutes to actually explode? As soon as that thing crashed and exploded behind the Falcon - BLAM! WHITE LIGHT! OUTSIDE SHOT OF DS EXPLODING!

You never had a 10 minute build-up (with the Death Star explosion) in ANH (remember the time factor was the torpedo travelling down the shaft to reach the reactor - once it hit, the whole thing blew up), so what they did in JEDI just seemed like these characters just CAN'T be killed no matter what happens - not even PHYSICS can kill them! So on top of the teddy bears, the constant "FREEZE, Rebel scum!... Wait a minute, the rebels aren't telling US to freeze... they just SHOOT us!" moments and everything else, how can we possibly take this seriously, regardless of Lando dying or not?

Ady:
I think that has to be a factor when editing JEDI:R; when the reactor blows, the DS IMMEDIATELY explodes... no secondary explosions, no flame coming out the hangerbays (maybe alarms being that they are under attack) - thus regardless of HOW Lando blows up the reactor, he would know that to go inside and destroy it from the inside that he SHOULDN'T have had the time to escape.

This would be the tough part: maybe with some "cantina-izing" to help Lando express what he knows is a suicide mission, as well as change the music to a degree to reflect the desperate nature of the Rebel's battle it would help to sell the death of Lando.

Some other ideas as to improving the serious tone: yes, Leia is hit, but maybe Han needs to get wounded as well - not straight out KILLED, but show most if not all of the main characters in a weak or wounded situation to push the belief that maybe they could ALL die.

I like someone elses idea of Han saying "I love you" to Leia like he expected to die and Chewie in the AT-ST blows up the 2 troopers and saves them - maybe instead of having the 2 troopers run up and say "Freeze!", maybe you could edit it so more laser shots start hitting around them after Leia is hit; thus, when Leia pulls out her gun, it is not in reaction to the 2 troopers so much as Leia saying (without words) that she intends to make her last stand; Han says his line, the 2 troopers run up and as they are aiming BLAM! they get destroyed; Han and Leia turn and look up to see the AT-ST hovering above them...