There's always a possibility you will be questioned by immigration. Things like where you come from, what you look like, the mood of the immigration officers a work that day, ... might all influence the decision of whether to pull you in for thorough questioning, but you'll always have to talk with them to actually obtain a leave to enter.
During this talk they will have access to everything the airline has told them (and much other information), so if they can see that you don't have a return ticket for anytime within the period your visa was granted for, that will raise suspicions. As they can probably also see that your previous application was refused, even a ticket that can be changed for a moderate fee might be enough to alert them, and staying privately does not count in your favour in that situation (if you were going to stay at a hotel, you would also need to extend that if you changed your ticket).
If you had a visa obtained rightfully and had travel plans in accordance with the application, I would think you were good. My advice would then be to bring (but only show if asked to) the documents you attached (or should have attached, like consent from your employer that you can go on a vacation for the duration) to your visa application (or a representative sample, i.e. if you attached 12 paychecks, bringing a couple is probably fine).
The problem here is that you have planned a trip significantly different from what you described in your visa application, that makes it possible to question whether the visa was obtained rightfully. And your travel plans are far from accordance with what was described in the visa application.
You run the risk of having the visa cancelled at the border (and as a result being denied entry) and get a note attached to your file that will make it harder to obtained a visa for the UK in the future. (Of course anything on the scale from being let in to this is possible)
The best thing to do is to contact the embassy (or consulate or ...) that issued you the visa, tell them that your plans changed significantly and hear what they say. And stop making dishonest applications for visas, if you get a visa (to anywhere) plan a trip accordingly.