Observations...
1) I am also going with lots of troops and a few Russ.
2) I too have 3 lascannon sentinels. Then again, I worry about enemy vehicles more than most players. I have the 3 sentinels, and a heavy weapons squad with either lascannon or rocket launchers attached to each platoon of infantry. I go nuts with heavy weapons, but looking at some of the tank parks and big bug armies Saturday, it is possible that one can't have too much tank killing. There are lots of mech forces floating around these days.
3) I found some cavalry I liked, but I can sympathize with your not caring for the desert look. I think rough riders could be effective, but they aren't apt to be used a lot due to taste in models.
4) I have a model that might work for Dedog, but haven't been into special characters. Thus far, I haven't put the model in my box. Have to try someday. Still, I'd rather have 2 sentinels or a heavy weapons squad than Dedog. It's just me.
Questions
1) I am going to try two company commands for giving orders with banners giving moral re-rolls. My platoon commands are also mixed use. They too will be giving orders at times, but they were equipped with a plasma pistol and melta guns for tough close in work as well. Neither are being geared for close combat. I'm hoping rough riders and sentinels will provide the counter charge for melee. I've some power fist models that I could play as commissars, but I'd like to see how the army plays without them.
2) I don't know that there is an obvious way you have to slant in terms of vehicles or infantry. Looking at the the tournament Saturday, it would seem either approach ought to work. My current 2000 point prototype list has just over 100 infantry, 15 rough riders, 3 sentinels, a battle tank and a demolisher, but the chimera heavy lists seemed to work, and I anticipate squadrons of Russ variants being tried too. With tank squadrons, lists similar to the old armored companies are apt to reappear.
3) I've moved my heavy weapons into support squads attached to my infantry platoons. I want the infantry squads to be all lasgun, to maximize the Fire by Ranks possibilities. Heavy weapons squads of bolters and rocket launchers round out each platoon. The idea is that each platoon ought to be self sufficient, but the heavy weapons don't have to shoot at the same target as the massed lasgun. I'm considering adding flame thrower and/or grenade launcher squads to each platoon as well. I've got the models, but not the points. I don't think my approach is the only possible one, however. If you have enough other tank killing stuff, you might not need it with your troops.
4) Ask Alan about Valkaries. They look like fun and seemed effective Saturday. They might prove very useful for missions where you have to penetrate the enemy's deployment zone. That's my current concern with my infantry heavy approach.
5) Armored Sentinels with Plasma Cannon? Hmm.... Maybe. I know my Eldar war walkers tend to die rather quickly. Lightly armored vehicles with lots of firepower attract a lot of attention. You might try it. There are enough marine armies around that something like that will be useful.
6) I have two Russ currently. One is designed to take on medium infantry, with battle cannon and 3 heavy bolters. The other is geared for heavier stuff, with a demolisher cannon, lascannon and two plasma cannon. Hopefully, the flashlights can handle the light infantry. I would look at the rest of your army then design your tanks to handle what the rest of your forces can't. There isn't a lot in a guard army that likes to play with terminators, carnifex or similar monstrosities. I have found the demolisher expensive but useful.
I generally stand and shoot my tanks. Thus, the sponsors make sense for me. If you tend to move a lot, they make less sense.
