More theories than tips, but could work

So, I would like to say this off the bat, I looking at mechanics and seeing how I could use them in interesting (some could say power game-y) ways.  I have thought about these a good bit and only really used one of them in an actual game.  I have 3 of these ideas, 2 of which use Sorcerer's Metamagic.

The first one that I actually tested was with the 5e School of Illusion Wizard.  It has an ability at 6th level called Malleable Illusions, which got replaced with Phantasmal Creatures in 5.5e for the Illusionist.  Malleable Illusion allowed you to use an action to change the nature of the illusion while the spell is still up (the new form still has to follow the same parameters of the spell though).  Now this can be quite affective with spells like Disguise Self for when you turn a corner and want to change your look without dropping the spell.  Now... I feel like the spells that benefit the most from this ability are: Creation, Dream, and Mirage Arcane.  Since Creation makes objects that can have a physical presence, you can first cast it to make a wooden bracelet and then turn it into a club or staff at any moment.  This would be amazing for stealth missions or general uses.  I remember using this during a one shot and it was really cool (and also backfired a little too).  I used an upcasted version of this (I think 8th or 9th level) and had it start out as a stone bead decoration on a bracelet.  We were on a ship and we needed to go under water into a cave to fight a kraken.  I casted Water Breathing on us and then turned the bead into a big stone pyramid that let sink faster into the water.  Later on during the kraken fight (I made bead into a stone javelin before the fight), I asked the other player I handed the javelin to to throw it at the kraken.  I was holding an action to turn the javelin into a really big stone cube to slam into the kraken.  When it was the kraken's turn, it proceeded to throw that stone cube at me and break the cube.  Now, seeing how that worked in that group I played in, you could imagine some of the stuff I could do with other illusion spell hopefully.

For my second idea, this uses the Metamagic option Subtle Spell.  Now, this Metamagic option does not have have any specification on the spell's casting time.  This is simple for spells that use a action, bonus action, and reactions, but it gets interesting when the casting time is a minute or more.  So, theoretically, you can subtle cast a spell like Legend Lore mid conversation with someone to learn about the thing they brought up or the person you are talking to.  If you multiclassed into something that gives you Glyph of Warding or Mordenkainen's Private Sanctum, you could cast this in someone else's place to give you more of an advantage before a fight breaks out.  I would say since you are still concentrating on casting the spell, you could get nocked out of it.  Oh, and since all spellcasters can ritual cast now in 5.5e, I am not sure how that could work with Subtle Spell.  I would just leave that up to GM's discretion, but I would think that it would probably not work or rule that you would have to be stationary while ritual casting and using Subtle Spell.

And for the final theory, this uses Subtle Spell and Suggestion.  Since Subtle Spell removes the verbal component, you could use telepathic communication (if you have it) give the suggestion rather than talking.  This could definitely be useful during stealth operation.  This one is definitely more concept, but I feel like this surely works mechanically RAW.

Please do tell me what you think about these and if they would actually work or not.

Most of the Malleable Illusions ideas you listed do not seem overpowered.  If anything they are in line with the expected use of the spell.  Nothing special.

Except for the throw a javelin bit.  That does not work the way you think it does, because you do not understand real world physics.  Listen carefully:

The normal, physical universe does not conserve speed.  It conserves energy.  Speed changes all the time, energy does not.  Example: if you shoot a bullet into water,  it slows down and converts it's speed into heat.  The bullet slows down, as would your stone cube.  The total energy remains constant, not the speed.  The cube you throw does LESS damage than the javelin because the javelin was sharp and the cube is not.

Could magic change this?  Yes, it could.  But neither the ability nor the feat say they change this.   Magic does what it says it does, nothing more, nothing less. The rest of the normal physics still apply.  A disguise spell does not also let you fly.  

Note, as a general house rule, spells and abilities that do not list damage should do LESS damage than a similar spell or ability.   Otherwise it makes spells not intended as attacks be better than spells designed to be attacks.

Subtle Spell is a wonderful, underused ability.  Note look at Counterspell and then think about what Subtle Spell does.  They do not know you are casting!

Suggestion should work - as long as they can still hear you even if you are not speaking.  Part of the spell description.  I do not see any obvious problems with your other listed use of the Subtle spell, they generally are in line with the intended use - to prevent people from realizing you are casting.   But for spells with a longer casting time, I would not be surprised if your spell is interrupted.   Especially as I doubt a DM would let you keep casting and also speak to other people.