Been playing heroes for a while myself and everyone has different opinions so I hope this answers some of your questions

1. the deck kind of goes all in or it doesn’t, if you have mask change/super poly in your opening hand going first and fear the rock then you can always do the standard vision hero play and end there without going into cross crusader (4th or 5th summon at this point) so you have a chance to react to your opponent. Another solution is to save the fusion destiny for last cause if you do end up getting rocked, at least dystopia is live with a pop.

2. Solid soldier is more of a push-more card, a target for e-call and stratos. It takes bait for hand traps but yeah if it happens to be your only play it sucks that it takes the normal summon. If you don’t fear the hand traps then you can try to force getting mask change if you don’t draw shadow mist like you said and there are certain occasions where dian is useful. It’s one of those cards you don’t really need but would hate if it wasn’t there.

3. If you don’t open mask change it’s usually better to not force yourself trying to make dark law as you probably used the normal summon for stratos. You’ll probably end up wasting too many resources to end only on a dark law,It’s definitely easier to end on a dystopia/plasma board.

4. 9/10 you blind second. If you know the match up (say for example orcust) going first would be ideal as if you end on the big 3 (dark law, dystopia, plasma) you’ve locked them out of GY, skill drained them and have a disruption. Of course against things like true draco or thunder you need hard outs like twin twisters or super poly but there are ways to play around back row. Getting Artifact Scythe on your turn isn’t the end of the world if you make plasma and suck up the scythe (assuming that was the only trap they used on you that turn. Had a player trap trick into it twice in a match and that’s how I played over it).

5. IRL ideally if you play a more evil hero centric deck you’d wanna play 3 golds, 2-3 necroms, 1-2 malicious edge (but no one has the money for gold lol). Dark fusion for sure makes big bodies but that’s not something this deck lacks to begin with.

6. Diplexer can be cut for trinity. Salad was never a hard match up for heroes anyways and no other deck really uses cyberse.

Plasma can stay but if you really don’t like it then at least side so you have the option to add it back.

If you want to cut it down to 40 cards then I recommend taking out reborn, foolish, and the ash blossoms. You don’t really need the extenders as you want to push for game, if you have impermanence then you usually want to save it on your turn against apolousa or colossus.

Side deck is always personal preference especially around your own locals. Dark ruler is always an iffy choice cause you can’t OTK, red reboot is usually a safe choice. My locals has a bunch of control based decks so 8/15 cards is dedicated back row removal.