will the pax get compo for that late arrival back to EMA
All the passengers on the return leg have as far as I am aware been given food and drinks vouchers down in TFS, but with it being about a 4.5 - 5 hour delay, the passengers won't receive any more compensation.
wonder why they didn't use the ALC based aircraft that you have mentioned so often
The ALC spare is only a 737-300. The problem with that is, the max capacity of the 737-300 is 148 (as Lord Spandex Masher correctly pointed out above). The 737-800 was almost full (just under 180 pax) on the return leg, so using the ALC airspare, would mean leaving pax stranded in TFS. The crew on the return leg will have now ran short of hours, so the LBA based team will have been drafted in as standby crew. The 757 will have on board the relief team (2 pilots + 4/5 crew), and will also have to have spare seats for the out of hours crew (2 pilots + 4 crew). It is also more favourable to the pax, as opposed to doing TFS-ALC, than an aircraft change, and a crew change, and then ALC-EMA. The strategy I talk about is mainly for summer and peak times, when such aircraft and standby crews would take much longer to source.