G'day.
MG, I'll see if I can answer your questions, though hopefully Oli and vW will also step in.
1. Some of the regiments with pre-1808 colours had enough to issue a stand of colours to their reserve battalions (earlier allocations had been five colours per battalion). There was a magazine article I read in the '80's (in Military Modelling or one of the wargames magazines, I think it was) that even suggested that the Silesian units did that. However there is no German-language source to support that claim, as Oli and vW have shown. Also, the colours of those regiments who did not lose their colours, but were not re-raised (eg IR Nr 19), would also have been available.
2. This is merely my flight of fancy, to give an opportunity to use some different flags to dress up the regiments. While the cavalry were carrying at least two flags that supposedly dated from 1713-1715 (they may have been copies, issued in the late 1790's, or have had extensive repairs done in that period), it's unlikely the infantry flags were in any condition to be carried.
3. Possible, but unlikely. The RIR battalions had very strong ties to their Stammregiment (parent regiment), with their Officers and NCO's coming from there. So while those worthies would probably accept the carriage of a flag from the Stammregiment, they'd be unlikely to accept a "home made" flag with any great enthusiasm.
4. Only the King had the authority to permit units of the royal army to bear colours and standards, or to have a unit issued with them. Flags issued by mayors/governors/nobles therefore could be seen as challenging his authority. Especially units from the Polish provinces annexed in the Third Partition, some of which had very recently been part of the Duchy of Warsaw, as well.
All the new regiments, including the LW, were issued new flags from September 1815 onwards. This included the fusilier battalions of the infantry regiments. The AKO of October 1813, ordering the removal of unauthorised flags, was never rescinded, to my knowledge, so it's extremely unlikely that any unauthorised flags were carried (at least where someone not in the regiment may see them). Even the 1st Silesians, who lost their colours at Vauchamps on 14 February 1814, had to wait until after Waterloo to receive new colours. And when they got them, they weren't the old-style, red-with-white-centres that they'd carried, but the new Model 1810 black-cross-on-white-field that was now the infantry pattern.
Sparker, here's some more (top two are both sides of one colour), mate, for some brave Brandenburg battalions:
Cheers.
Dal.