Questions about Xbox Game Pass

1. Not as good or as safe as the official sources.
2. Yes, subs stack.

A cheaper way is to get xbox gold, you can buy the 12 month one and then get 1 month of gamepass ultimate, and the 1 year of gold gets converted into gamepass, so you get 13 months of gamepass.

All game pass and xbox gold variants are accumulative. However, if you activate a lower tier of the sub you have (for example, if you have game pass ultimate and buy a regular game pass for console code), you get less months (something like a 12 month card will get you 8 months or something like that, you can google for more precise information on conversion rates). Best option is the one drbeckett mentioned.

There's also an option (at least here in Italy), that requires some patience. If you use Bing as a web browser, you can obtain points on your microsoft account. And other bonus points, with easy daily tasks, as you may see here. And after a while you can convert them points in some other stuff, including usually months of GamePass.

Have used the sites that you mention for several years and never had any issues or problems with them. Have also used cdkeys which again is 100% reliable. Bought gamepass from them as well as Microsoft and am subbed till the end of next year. Also on a side note it's worth doing the microsoft rewards as well as becoming an ambassador which if you do all the missions and get upto diamond tier they reward you with a free month of ultimate game pass. So in effect you could get 4 free months a year :)