Yes, 900 hours limit. (As my lot send us around in taxis all over the place, there is very little chance of getting near 900hours, more like 500-600)
Yes, paid instruction counts for flight time limitations. (Unpaid below 1600kg doesn't, I think).
Yes, airlines do allow it, usually just ask the fleet manager or chief pilot for permission.
The CAA now frown on paid instructing during your days off or certainly the FOI that my company deals with certainly has taken a firm stance on the matter. It is a clear sign of how unimportant the CAA see the development of G/A in this country and with the Multi Pilot commercial licence on the horizon, it is clear that the CAA now see that G/A plays no part in the development of our industry as a whole.... How wrong can they be?
As time goes on I have been requested to jump through one hoop and then another as dictated by the CAA and now I am forced to instruct for free - which does not bother me personally but there will be some out there who would like/need the money! Paid instruction on a day off is no longer counted, as a day off which is ridiculous! I find instructing falls into the category of being a pastime and not a job or else I why would I be doing it my own time. If the CAA are so worried that I may tire myself out whilst instructing then why can I still be (for example) a adventure sports instructor on my days off and tire myself out doing all manner of lively activities yet I can't do a single trip in a light piston single? It is just ridiculous! Do the CAA actually think that we will be willing to flog ourselves to death on our days off so that we start work absolutely knackered... or perhaps those in the CAA that have flown have forgotten the responsibilities that are placed upon me when I sign the skippers acceptance at my 'day job'. I am certainly not going to risk loosing my little green book for being too tired due to instructing when I am on a day off!