On the contrary, I don't believe that the public in either France or Britain would have felt that way. Poland would have invaded Germany and as such, Poland was the aggressor. The Poles couldn't even argue that they would do it because of Czechia, because they would have had to invade right then and not that much later.France was by no means as committed to a peaceful course of action as was Britain. If Poland attacked Germany in the summer of '39 it would be very difficult for Britian and especially France to stay out of it. The Poles could declare that they were going to war because Germany had invade Czechia some months before and had also withdrawn from a nonaggression pact with Poland. The public in France and Britain would unquestionably have felt the Poles to be in the right and be quite ashamed of their own governments if they stayed out.
Withdrawing from a non-aggression pact isn't a cassu belli in any case. Moreover, France a Britain were very reculant in helping Poland in OTL, why would they do any more if the country they are meant to defend starts attacking, even if they should side with Poland?
Additionally, with the events of WWI fresh in their minds, I doubt that the Frensh would have DoWed Germany on their own.
This would apply in OTL, but not when they would help a country which is attacking another country. However, I don't know if the treaty simply guaranteed the western border or if it guaranteed the border in case of an attack.One can say that the treaty would not have "obligated" Britain or France to go to war if Poland attacked - but they had recently guaranteed the Czech's new borders and certainly failed to act on their obligations. The only possible reason for Britain and France to go to war in 1939 was that failure to do so would have meant the fall of their govenments in the next election (possibly even sooner).
Seeing how willingly the OKW commited itself to the different invasions and how long the cast majority of the officers followed Hitler, I don't think there'd been the slightest chance this could happen.There is another dimension to an invasion of Germany. So far this discussion has ommitted the feelings of the German general staff towards the Nazi Party. There is a good chance that the generals would have used an invasion (by anyone) as their chance to get rid of Hitler. They may even have kept a front at a standstill inside of the German borders until they could negotiate a proper settlement with all the parties involved.
The flaw in your assumption is, that there were enough ethnic germans in the western part of Poland to make it impossible to move troops secretly. An attack by the Poles without the element of surprise would've been a lot less effective. Even with initial successes, they would still face the battle ready Panzer divisions. Without surprise, it's very hard to imagine that the Luftwaffe would loose a lot of aircrafts on the ground or that there would be even a couple of sunken ships. The loss ratio in aircraft would be even more in Germanys favour as the Polish AA units would play a much less important role here, at least initially.Would invading Germany have helped even if the above didn't pan out? I believe it might have been very beneficial - especially if we recall that this thread was started under the assumption that Poland had obtained the complete German plans. Given that they would have known troop dispositions and been able to direct their attacks against the most vulnerable German units - they could also have likely destroyed many German aircraft on the ground and even sunk a few ships before Germany could react. Considering that the battle of the Bzura saw a relatively successful Polish advance I see no reason that a number of simultaneous Polish offensives, made with full knowledge of German plans, would be any less successful.
I still don't see how the Poles would've managed to gain more than first day successes against an outnumbering army with a lot more and better equipment. Courage isn't bullet-proof.