When you enter combat, you have to choose the weapon you are going to use for the entire combat. If you choose a Spear instead of Hand Weapon (for example), you use the Spear until that combat is resolved (one side or the other is dead or has run). If you have a magic weapon, however, you may *only* choose the magic weapon. [There is a special rule for Brettonians allowing them to charge with a lance and switch to other weapons, but that is an exception.]
Temple Guard numbers are kind of a matter of personal choice. I see lots of people with 16 Temple Guard (giving you a 5x4 for max ranks) but not many with 20 Temple Guard (giving you 6x4). I think this is due to a couple things. First, the Temple Guard is really, really defensible and will almost never run. Second, your Temple Guard + Slann unit usually isnt going to need or even want a large front (most of the time). A larger front makes it harder to move and increases the number of enemy that can be brought to bear. In general, you want the Temple Guard to keep the Slann safe and to provide a very steadfast anvil, then you bring something into the flanks as the hammer. A 16 strong Temple Guard will usually manage that for you.
Again, the number of Temple Guard is personal choice. I'd cut it to 16 and invest the points elsewhere, especially since you have a large block (6x3) of Saurus Warriors on the board as well. You'll be able to use them to help minimize risk to your Slann.
And no, your Slann cannot be challenged unless he is in the front row. If that happens, things have already gone very, very poorly.
Lizardmen are fairly weak in Shooting. We arent as terrible as some *cough*WOC*cough*, but we dont have very strong options. Our Skink Skirmishers are handy here and of course we have some ranged firepower on our Stegadons. Salamanders are pretty darn handy for Shooting. The panic tests are just icing on the cake. Razordons also fall in the Shooting category, but I'm of the opinion you can get better results from a unit of Skink Skirmishers and for less points (there's a lot of debate on this point, you can check out some other threads for more viewpoints). Even little units of 10 ranked Skinks can toss out their 8" poisoned javelins for a little harassment while they are helping control movement on the board.
In general, if you are wanting ranged, I'd go strong magic with maybe a couple Salamanders for support. Our behemoth Magic phases, along with the unlimited range of Lore of Heavens damage (from our Skink Priests) provide the bulk of our ranged attacks. Skirmishers are extremely useful against anything with bad Armor Saves (Toughness doesnt matter thanks to Poison).
So, in summary: Lizardmen arent really strong at Shooting, but we can have very powerful ranged if you focus on Magic. In general, you will either want to use protection against ranged to get your close combat troops into battle or heavily focus on magic and supplement with our moderate ranged abilities.
Engine of the Gods is a tough choice. Looking at your particular list, I'd be tempted to stick with the normal Stegadon or maybe even an Ancient Stegadon. You have great Magic as is, and the Engine of the Gods changes the way you will play your Stegadon. As your list stands now, your two hammer units are the Cold One Riders (which I assume are going to be off taking care of enemy support units) and the Stegadon. If you are using the Engine of the Gods, you may be less apt to charge into battle because the Skink Priest is just so damn squishy. Once he is gone, the Engine basically becomes an Ancient Stegadon without Giant Blowpipes.
For your list, I'd keep the normal Stegadon. This one is, again, very much a personal choice thing.
Edit: Haha, looks like Barotok beat me too it. Forgive the repetition.
