Polish cavalry brigade was naturally more mobile than an infantry division, at least on tactical scale (wagon trains made Polish cavalry units slower); The first phase of the campaign in the Corridor, from Polish POV, is based on manouver - Polish units make token resistance and withdraw. That is the job for mobile units, i.e. cavalry. The second phase is hard defense of the main line, where mobility is much less important. That is the job for bigger, but less mobile infantry divisions. Using cavalry units in the Corridor you have at least a chance to withdraw them and then you have both infantry AND cavalry on the main line. Using infantry units in the Corridor means loosing them for what is supposed to be only a token resistance.
There is a reason if I suggest to use a reserve division or two there, and not the active infantry divisions.
As to the "token" resistance, yes, it has to be token, but not
too token. The OP posits that the Poles know about the german plan, but they don't know what the French and British will do. If the resistance in the Corridor risks being downplayed as local disturbances of the knid that happened in the Sudeten where the Czechs, eventually, did not fight, that's a serious liability.
The other aspect is that, sure, the cavalry brigades would be the units that are able to wage a fighting withdrawal. But they also are the best suited units for those counterattacks in the German flanks. That's what I want them quickly coming back to the MLR.
But the air recon is still up there, although I admit it was far from perfect. Poles indeed tried counterattacks, but they were usually poorly coordinated. Also Poles need to find the corect place for a counterattack and then have enough forces to make such a counterattack - that requires good recon and communication, which was a problem in Polish Army. And I'd like to remind you that after initial success Poles did loose the battle of Bzura.
Sure, all true, and these counterattacks also will be eventually contained, the Germans are just too strong.
But if they are contained
after they have forced the armored thrusts to either withdraw or at least stop, that's a success.
The overarching point is that I'm playing for time here.
The reasoning is the following:
1. if by September 15 the Germans at least
seem to be mired in difficulties, then it is possible that the ever-cautious Stalin says, "oh well, let's wait another week", and
2. if Stalin says so, then the French have no excuse not to carry out a renewed, larger offensive around that time, and
3. even if the French are, as predictable, half-hearted and not very successful, that in turn will make Stalin even more cautious, the German generals even more worried, and it will also be a little morale boost for the Poles, and
4. without a Soviet intervention, and with the Germans having to reinforce the West, there is half a chance for the situation to congeal until mid October - when the weather turns bad, which favors the defenders. Around this time, the Germans will be running out of 37mm rounds for their tank and AT guns. The Romanians might be pressured to let supplies get through. A slightly bigger slice of the Saar will be in French hands. Guerrilla operations might be going on in German-occupied territory (as per OTL, save that in OTL they had virtually no external support and no hope). Stalin might start considering another turnover in the People's Commissariat for Foreign Affairs. And so on.
I don't know. Poles had in the region 7 infantry divisions and 4 cavalry brigades (assuming Wyszków Group is mobilized) + some ON and fortress units.
German 3rd Army had 7 infantry divisions, 1 cavalry brigade, 1 armoured division (Kempf) + fortress and Grenzschutz units. However Germans had also support from Luftwaffe and could count on Guderian's XIX Motorized Corps (from 4tgh Army) which would have come quicker with smaller Polish resistance in the Corridor + some divisions from Army Group North reserve (like 208th ID).
Indeed I don't expect the Poles to conquer East Prussia. I weant them to make a serious dent in Eastern East Prussia, so that, instead of fighting a defensive action on their own soil at the German terms, ceding them the initiative, they fight a diversionary action on German soil, having initiative. Sure the Germans can move reserves, and may troops from 4. Armee, to contain and eventually beat back the Polish operation - but that's the point. Plus some good propaganda for taking German territory and bringing the fight there.
Note that:
1. of the 7 IDs of 3. Armee, 21. and 228. attacked towards Torun. They will still need to go there, i.e. in the wrong direction. The troops we have to look at are those that attack South towards Mława, and those that hold their ground in Eastern East Prussia;
2. Kempf's unit had tanks - that's what we can say about its being an armored unit. But calling it a Panzerdivision is an exaggeration. It had one Panzerregiment, which included a dozen real tanks (Pz III and IVs), some 80 light tanks (Pz IIs) and some 60 tankettes). It's somewhere between a standard light division and a true Panzerdivision, with the additional drawback of having SS troops for its motorized infantry.
3. as to the quick arrival of 4. Armee, yes, we're talking about less resistance in the Corridor, but we also have that wonderful foresight. Demolish the bridges, crater the roads, etc.
4. if you throw in just two of those additional fully mobilized Polish infantry divisions, things might just work.
Another thing is that Germans had the initiative of attack, which meant they could have concentrated their forces,
Yes, that's the point, taking some of the initiative from them. And yes, they could have concentrated their forces, but they didn't - I'm assuming they attack as per OTL, while the Poles have foreknowledge.
and Poles had much smaller margine of error - should Germans broke their line on Narew, whole defensive plan would have gone down.
I'm not sure if Group Narew was strong and mobile enough to attack, do some significant damage or at least make some confusion in German ranks (allowing Army Modlin to withdraw in peace from Mława) and then withdraw themselves to their main position on the Narew River.
In OTL, the 3. Armee attacked South with five IDs, the cavalry brigade, and Kempf. I say delay and then stop them with Armia Modlin, with its historical strength (2 DP, 2 BK, 1 ON brigade) augmented by two of the fully mobilized DPs now moved up from the East or reserve. Meanwhile, Grupa Narew attacks to the East, with 2 BK and 2 DP, faced initially by Grenzschutz units and the 206. ID (a 3. Welle division, BTW), to which the Germans may add the 208. (ditto).
I say the German initial drive onto Mława cannot go better than in OTL, while their defense to the East of it can give them some headaches.
Well, Slovakians had their own air force, IIRC at least 60 B-534 fighters; I think it would have been enough to protect Bratislava.
Sure, after a first surprise attack that will in all likelihood get through unscathed or nearly so. I don't suggest a continuing campaign there.
The Germans might make some gesture sening 1 or 2 AA artillery units or even a squadron of fighters (doubtful), which wouldn't have changed that much in the Polish campaign. In German opinion Bratislava might burn, they truely do not give a damn.
Yes, and one of the points is showing the Slovakians, and any other German actual or potential ally, just that.
And I'm still not convinced about costs of such an action, both political as in equipment. I doubt it would have been worth it. If Poles were to bomb something, it should be something German.
The French will hate that, in fear that their cities will be bombed. But there's not much they can do about that; their attack will be half-hearted anyway, but if Stalin is sitting on the fence and Poland still stands by mid-September, they must do it, exactly because of the political cost of not doing it.
And it the Germans do something stupid, both in retaliation and because things are getting worse in the West, like bombing a logistical rail node behind the Saar offensive, so much the better. The French will clamor for air attacks on Germany. It's a gain, for the only country whose cities are being bombed ruthlessly at this time in OTL.
The cost in equipment - who cares? It's not as if those bombers did much of significant in OTL, and indeed not a small number simply interned into Romania at the end of the month. So expend them.
As to bombing the Germans - yes, I like the proposal by Rich, one can also send some of the older and shorter-ranged ones, the Karaś, to hit the Stukas' airfields after they have just landed. This doesn't necessarily exclude the propaganda raid on Bratislava.