Since speed of the entire unit is a concern, what speeds are considered to be the standard movement speeds of the various units? Will a force made up of 4/6 and 6/9 mechs travel at speed 4 or speed 6? For that matter is the DVS-2 considered a 3/5 mech or a 3/6 mech due to speed demon?
Slowest cruise/walk speed. Flank speed can be used for a bit, but it carries a MUCH higher risk of breakdowns, as per the Techmanual fluff. The DVS is 3/5; Speed Demon doesn't really apply for strategic movement...only tactical movement.
I realize that I cannot use WP to get jet fuel, but will I have to spend WP to acquire the chemical fuel for the ICE engines?
Gas is something that you have to have an "excuse" to buy with WCP (both ICE and ASF fuel). That is, go do something to earn you some gas, and then you can spend WCP on it. Ammo is only available to be purchased because you have all those cargo haulers (and, frankly, because I'm not forcing you to go the next several months with only training ammo in your bins). After a while, if you have done nothing to "earn" ammo, then you may no longer be able to buy it with WCP. However, that'll be a little while, given that you've got 50-ish tons of spares. Note that we're only really tracking this in an incidental way; armor and such isn't going to have to be "earned", since it's a given you can salvage it off of any given battlefield. Basically, I'll be taking things off or putting them on the "List of things you can buy with your WCP" according to the circumstances of the game. Contacts, Connections, Scrounge rolls, and similar may also have an effect upon this...once you're in a position to logically use them. You can't scrounge for spare parts or meet a connection when you're about 500km away from civilization, after all.
Also note that you cannot purchase new Mechs with WCP at this time. All ICE vehicles have at least 2,000 km worth of fuel endurance, so you've got some time before it becomes an issue.