1

One needs to distinguish between operators and agents. It is common for there to be only one operator on more niche tours, because there just isn't the volume to support more than one. A few very popular tours do of course have direct competition, like Golden Circle. A large range of agents may all be offering some operator's particular tour. So that is why you go can go on various agents' sites and there turns out to be a large overlap between what they are offering. It doesn't follow that they are all the same company, but it does follow that you can end up getting much the same thing whichever you book through.

2

Iceland rovers and mountainguides have merged into one company, and they are a leading tour operator, and has been awarded for their tours. Absolutely a trustworthy and professional in everything they do.
Icelandictravelmarket is NOT a tour operator, but just a booking center (wholesaler), and sells tours for different operators (unneccessary middlemen). Actually, you could be picked up by mountainguides if you buy from ITM.
It is important to know if a company is the real operator or a booking service, specially if it comes down to liability questions.

3

Thank you for the advice! They do seem to be a great company and I was just hoping to confirm with some personal experiences. Thanks much!