Ok, Mashing has a very specific meaning, it's the mixing of malt and water at the right temperature to cause enzymes in the malt to convert starch into sugars.
Crystal Malt has been mashed, it is done inside the individual grains (called corns of barley), then the malt is dried and roasted hotter and longer than for base malt. The palest crystal malt is Carapillis or Carafoam (same thing) they are not much darker than base malt. The darkest would probably be CaraAroma. Because they are already mashed crystal malts just need soaking in warm water to extract the good stuff (called the goods).
Crustal malts are made to add flavours and colours to beer that you cant get with base malt, they as above, tent to be full of less fermentable sugars (Dextrin's), what are called Milliard products (provides most of the colour and caramel flavours). As a rule the darker a crystal malt is the less fermentable extract you will get from it and the more flavour.
As mongey says 500g in a standard 23L brew could be a lot (even too much) it is going to depend on how dark the crystal malt was. 500g of really pale crystal probably wouldn't be too much (CaraPillis, CaraHell, maybe UK or Aust Cramalt). If you used UK or Au Medium Crustal or say German Caramunich you would really notice the beer being darker and sweeter. If it was Dark Crystal or something like Caraaroma it would be pretty in your face and for a lot of people it would be way too much.
All malt will have a fraction that isn't soluable, no matter what you do with it. There are lots of ways to express this and without getting too scientific about it, lets talk about malt as is and assume that its crushed resonably.
As is includes the moisture content which doesn't add anything to the gravity of your beer, a reasonable crush is one that lets the water get at the insides of the malt and extract the goods. Think about Coffee, a coarse grind is good for a stove top boil, a medium grind would work best in a French Press (Plunger) and an Espresso machine will give you the best results with a fairly fine grind. Mind you too fine and tamped too hard and even a 9 Bar espresso machine wont push through the coffee, too coarse in an espresso machine and you wont get much flavour, the hot water/steam goes around the grist not through it pulling out the flavour, so yes grind matters.
All malts come with a measured yield (the amount of goods you could get under lab conditions) its expressed as a percentage (outside the US) a medium Crustal should give something like 65%, under home brewing conditions (70-75% Lab). This means that your 500g of Crystal should have added 500*0.65=352g of extract.
The kit weighs 1.8kg (1800g) it is partly what are called malt solids (extract) so the kit adds 1800*.8=1440g
Dextrose has some water (moisture) and is 91% solids so 1000*.91=910g of extract.
Add the ~350g from the Crystal malt and you have a total of 1440+910+352=2700g
Of solids in solution, say its in 23L
Brewers use two methods to measure how much stuff we have in solution. The Specific Gravity (SG) scale which is times the mass of water i.e. 1L of 1.050 wort would weigh 1.050kg (1050g). The Plato scale (oP) which is percent Weight/Weight if we dissolved 125g of sugar in enough water to make 1000g it would be 12.5% by weight sugar or 12.5oP.
We can put these two bits of information together into a formula that is very useful.
Mass in Solution = Volume * SG * oP
We in this case know the mass in solution (well pretty close) and the volume.
There is also a relationship between SG and oP roughly SG=(4*oP)/1000+1
The easy way to work out the density would be to stick the formuls and the information we have into a spread sheet and use goal seek to work out the SG and oP, which is exactly what I have done.
Remember to keep all units the same, use g or kg not a mixture, water is close enough to 1kg (1000g) /L (I'm going to do it in kg and L)
2.700=23*SG*oP, gives an SG of 1.0449 or 11.23oP
If you added another 500g of Sugar (far from clear what you have actually done)
3.200=23*SG*oP, gives an SG of1.053 or 13.2oP
You can see why I would say that 1.030 and 1.060 are both impossible answers.
A bit of basic brew maths can be very helpfull.
Mark
