Ok, so in response to chaos melee it must first be noted that chaos is very effective against IG, probably the best race against them. You forgot to mention khorne marines in t2, those guys are so quick, and their specials hurt guardsmen so much and their pistols tear up chimeras
Now, if your the commissar play defensively with him at first keeping him back, near your guardsmen, you want to focus down heretics first if they are coming out alongside the chaos lord because they do more damage early on, you do want stomp ASAP, but you also want the power sword soon after this is because the special ability on it absolutely shreds heretics and it buffs your guardsmen's DPS so they can shred the chaos lord. Also, if melee is about to reach your guardsmen retreat them and then execute them when they are away, they will stop retreating at a safe distance and unleash double their normal damage on the heretics, killing them extremely quickly.
The inquisitor can do a similar effect against melee charges with her brazier, cast holy pyre in front of your guardsmen meaning any unit that runs up to melee (especially heretics and the like since they have so many models) will take a decent amount of damage, but more importantly will deal 30% less damage. Then you hammer of the witches and focus down the chaos lord.
Lord general well, you just want to help focus stuff down with him, he can also usebuildings to great effect, plus he can support catachans fantastically with med-kits and he counters grenade launcher heretics with grenade launchers of his own.
I would also recommend getting a heavy weapon team to really stall that advance of theirs, if they got havocs, it will make havoc playmuch less effective as they won't be able to push those havocs forward whenever they want. They might get raptors to counter this, but if you stomp the raptors then fire on them with both guardsmen and have a holy brazier/power sword hero nearby the raptors will die very quickly. If your the general I wouldn't recommend a HWT until you get catachans because you won't have enough damage to force off raptors or similar jump units. Also if they get havocs you can counter that with spotters, if they get double grenade launchers you can counter that with catachans (ol reliable really hurts the GL tics, and the knockdown lets you advance), if they get raptors and you can't already counter them with your hero (they might have just too much melee, say 2x tics, CL and raptors) well just throw in catachans too, catachans will destroy raptors in melee and can knock heretics all over. But yeah, the chaos lord is extremely strong early on, and especially in 3v3 where your forced to fight him head on, it will be rather hard to win the first engagement.
Regarding doubling, if you're on the bottom lane you really are risking a lot by going top. That's why the bottom lane is regarded as the "gutter", you're basically just there, left alone to do your job (defend/take power). The only time you should really support top with a full army is if you have just forced off your opponent, but even then it might be better to just push your lane and destroy his power. It depends. Also other races like space marines have units like scouts who don't contribute considerably to engagement unless upgraded, getting a second scout squad and using it solely to cap the middle even though your not on that lane is very very effective, it can force your one of your opponents to split their army which makes it easier for whoever has to fight that guy. IG unfortunately don't really have a unit like that, they kinda need all their units together because their synergy is that high.
The IG commanders are quite unique, the inquisitor is called an offensive hero, but really she's more supportive, she shouldn't ever be leading the line, not unless she has her shield upgrade, but that's seldom worth it. I think IG pressure and pushes in general, in t1 are based around sentinels. Sentinels are the pivot of your pressure, supported by catachan ol reliables or spotter artillery shells. You want your sentinel to tank damage, you want cats/spotters to negate suppression, double repair the sentinel, focus down opposing suppression. You're leaders are there to support you. The inquisitor can HOTW enemy suppession squads (on their key unit, the one with the actual heavy weapon) she can HOTW enemy heroes, immobilise melee with her crossbow, tie up heavy ranged squads, or she can fend off melee with her brazier upgrade, but she is not really meant to tank, the sentinel does that job. The commissar is the same, you want to send in your sentinel first, always with guardsmen support behind it,then after they unleash their ranged fire on your sentinel, ol reliable their suppression team, activate lead by example on your commissar and tie up the suppression team, support your commissar by advancing the sentinel and stomping any melee that try to attack him.
In t2, IG pressure revolves around the use of chimeras or ogryns. Chimeras I find are very effective against races like chaos, space marines, grey knights and eldar because these races will often get suppression teams and the chimera negates that and forces them to get some form of AV, but most forms of AV 2x guardsmen can actually out-repair. Then if you discover they have a lascannon you can deal with that in numerous ways, catachans, spotters, missle launcher sentinel (attacking ground) or a manticore. Your leaders then support the push, they don't initiate it. Ogryns also work if they didn't get suppression or if you're against an enemy who lacks plasma ranged damage, the ogryns are extremely tanky and synergise greatly with bionic eye( commissar), mek-kits/stabilisers (general) silently( inquisitor), they can tank most of the damage for you while also countering melee while you use your other units to again deal with suppression.
In t3 the pivot unit becomes the leman russ and I'm sure you get the idea now. Sometimes you can be in a really bad situation where you just can't push against an enemy with a vehicle without it likely dying, but in such a situation I think that's when a manticore is good choice. Get the servo-skull if your the inquisitor for the sight radius, or stormtroopers otherwise and infiltrate them so you can get sight radius on their army, soften them up by manticore barraging their key units-lascannons, suppression, artillery. Then lead with your pressure unit. The inquisitor and the commissar certainly can tank, but IG have better uses for those heroes and the heroes can't do it as well as other race's heroes can. The commissar can flare suppression and then charge in with lead by example and stubbornness, but he'll die much quickly than a chaos lord under khorne worship would, the inquisitor can infiltrate herself with silently and then charge in with her rosarius on, but again, she would likely serve better behind her pressure unit, supporting it. The general shouldn't ever lead the charge as such, although early on he can certainly be useful just tanking damage tying up melee, but once he starts getting abilities he is far more useful at the backlines buffing his entire army.
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