Electronic – Absolute maximum ratings interpretation

Absolute maximum ratings are there to tell you what is (pretty much) guaranteed to damage your IC if exceeded, they are not recommended for normal use (notice the "at these or above..." in the first clip below)

All decent datasheets will have recommended operating conditions, which are the ones you want to go from rather than the absolute maximum values. For most small micros, 15-20mA is a typical maximum operating condition.

In the datasheet for your uC, it gives a table showing the OH and OL voltages over current, which both range up to 20mA. Note the voltage rise/drop at 20mA and you can see exceeding this will cause the OH and OL levels to go out of spec compatibility wise (example shown for 5V supply - things are different for 3.3V and 1.8V)
For example you can see at 25°C, the output low voltage is at around 0.5V @ 20mA (more with higher temp)
So, I would assume 20mA max for the outputs (at 5V - see other tables for 3.3V and 1.8V). Also take note of the little notes under the tables detailing the guaranteed hi/lo input values and combined current ratings.

Basically decide on your operating conditions (temp, voltage etc) read all the tables carefully and do the math to make sure you keep within specs.

If you are planning on driving some LED displays, either use an appropriately rated driver chip or add some discrete transistors on each pin to drive the LED. Unless you are only using a couple of low current indicator LEDs, this is generally the way to go.

The cores in the transformers do not behave linearly. In general you can use less than the specified voltage, but not (much) more. If you connected a 120VAC transformer to 230VAC you would get smoke because the transformer core would saturate, the effective inductance would be less, and the current would be limited only by the copper resistance in the winding. This is actually a useful 'feature' since the flux is limited by saturation the output voltage is also limited and the transformer will greatly attenuate large spikes on the primary.

If you use a 230VAC transformer on 120VAC and stick to the rated output current you'll have no problems with smoke, however the transformer will be heavier and more costly than the cost-optimized design (it will likely also be a bit more efficient than the cost-optimized design).

Putting transformer primaries in series (unlike secondaries) is not a great idea. It may work in some circumstances, though the tendency will be for one transformer to run warmer than normal, so I would not recommend it in general.