Blood war ends, upper planes under siege - consequences?

Well, while I think that most likely that at least a few good deities would aid in a defense against fiendish attacks, I don't see why petitioners should be evacuated of where they should be evacuated to. Firstly a petitioner's home plane is the only place where he can die without being erased from existence and secondly the goods need followers to exist, not petitioners.

I would say that the fiends find out how hard it is to fight a war of conquest against motivated natives with equivalent technology and similar numbers.

However, let's check our assumptions. Let's say that devils are roughly equal in number to archons, and yugoloths to guardinals (I suppose the gehreleths are staying out of it? or not?), but demons well outnumber the eladrin and remaining groups. Thus we'll give the fiends numerical advantage over other exemplars.

However, the petitioners (the unpromoted petitioners, that is) can't be moved off the Lower Planes to fight, and if they are moved they'll be extremely unmotivated. Or will they? If a petitioner dies off-plane, he suffers oblivion. Might an unpromoted petitioner of the Lower Planes prefer this to his damned existence? Alternatively, could a petitioner convert postmortem through the use of /atonement/ spells, and thus be stolen? That would be a fascinating coup.

The technology of both sides, in their equipment, native abilities, and magical and deific abilities, is roughly equal on neutral ground. But if you're using the old 2e fluff, one of the key problems of crossplanar religious wars was that priestly abilities decreased in strength as a priest went around the Great Ring. In 3e, you have the fact that the Upper Planes are mildly or strongly good-aligned, and so the powers of Good will enjoy at least a small edge over invading fiends. There are also the features of each plane: Elysium's traveler's travail will make invaders most unhappy for a long time. So in terms of local knowledge and technological "level," we must give the edge to the celestials.

Regarding that neutral ground, by definition the middle powers in Mechanus, the Outlands, and Limbo will remain aloof unless the balance becomes tilted too far. However, among the vast populations of mortals, it's certain that the gods of the Upper Planes enjoy a clear propaganda advantage. Higher-class demons may be immune to less than holy weaponry, but the hordes of lower-class demons are quite susceptible to a hail of commoners' cold iron arrows.

The Princes of Elemental Good and Evil will probably mobilize immediately. The djinn and efreet will also take sides, but will primarily direct their energies at each other, so little will happen between them except an intensification of the combat in Smoke, unless one side or the other obtains an edge. Zaaman Rul may be promptly wiped out, though, leading to a firm alliance between Fire and Evil, forcing Air and Water generally to the other side. The marids may not be thrilled with that, but it will be politically expedient.

Eyeball summary: fiends have edge in numbers, surprise, and mobility (the latter two because they are on the offense). The celestials have the advantage of home-ground knowledge, superior abilities in their native planes, and public sentiment. Each side will maneuver to bring its strengths to bear.

Thus, I think the shakeout will be as follows: an initial incursion directly in to the Upper Planes will fail due to the defenders' advantages. Continued skirmishes will make the settled regions of those planes more fortified. Elysium will be fairly free of trouble, and Ysgard, with its resurrection ability, will have no trouble. Arcadia's weakened institutions will find themselves beleaguered and the front line will shift to the Lawful side of the Ring. The Outlands and Mechanus will become the front line for most of the combats. The modrons will defend the plane but refuse to side with Good or Law, and will likely hunker down and fortify. The formians, fighting the disruption of their home at first, will almost certainly return to the side of Good.

I think Arcadia will lose its epithet of 'the peaceful kingdoms' and become a plane of paladins and lines of fortress-cathedrals. Its primary danger will be rot from within, if the devils and yugoloths can manipulate the leaders into harshness and cause the rest of the plane to follow Nemausus into Mechanus, where the fiends have more equal terms. The Lawful side of the Outlands near the Upper gate-towns will be the primary front line, with secondary fronts at the Upper Chaotic gate-towns. I fear the Beastlands may experience significant trauma; the animal lords are powerful, but not well-organized or of numerous followers.

Slain fiends do return, yes.  But only after a wait of 99 years.  For all practical purposes on a short-term war plan, each side suffers casualties when their troops are killed, save for those summoned via spell.

Depends on how 'smart' you want to make it, I guess.  If you're going somewhere with good or at least neutral intentions as to what you want to do when you get there, you'll get their fastest if you take the route that presents itself as you help those in need on the way.  If you avoid opportunities to help people, you won't be taking the fastest route to where you're going, and if you intend to do bad things when you get there, you may not be able to get there from here.  The plane will sure be sending a lot of other well-intentioned do-gooders your way.  Meanwhile, try to keep an invading army of foot-soldier demons together when they're mainly interested in killing each other and pissing off anyone else they come across.  ;^)

Theorically, blood war balances the multiverse. Without it, the lower planes are more powerful than the upper, so Rilmani would decide to act to serve the balance. Monster Manual IV includes the rare type Proxy of the gods of neutrality, the Concordant Killer, so an increasing army of those could aso be included with the Rilmani.

I don't know very well the planar chant, but who is imprisoned in Carceri? Maybe could be interesting an epic event of liberation of the ancient Titans of Greeks to help against lower planes.