In USA due to smog rules during the winter the gas makeup is different then in the summer since engines run richer in colder weather so more O2 is in winter gas and make have more Ethanol ( corn)which produces less MPG then summer grade which has more gas then corn.

With Boost and if over 1.2 grms/cyl as example which is the max for timing table then PCM just uses what the highest value its table supports.

Being engine is at 100 or 105 KPA still means engine is in WOT and O2s are ignored and so are the fuel trims so its simply functioning on what PE tables values are

With boost that means both PE tables of RPM and temp have to be adjusted.
Not sure what you mean but IFR functions up to 100 KPA ( as per LS1edit) but other tuning tools only show table to 80 KPA which I think really for them is vaccum values

Again as to fue trims, STFTs are what PCM adjusts IFR and not LTFTs and if STFTs are close to zero then its a mute point as to effects to overall AFR.

I think you may be looking too much into this for as long as NON WOT has fuel trims close and during WOT you have good timing and little or no ping then fine tweaking tune buys you nothing.

Consider EPA (smog) they feel no problem as long as fuel trims do not exceed -13 and +22 so in your case with +/-2 of fuel trims your pretty well tuned in.