Law of Identity. "A" cannot equal "non-A." There is nothing in the context to lead one to conclude the promise is anything *but* a blank check. My premises are proven by your acknowledgement it's not a blank check. Your problem is to prove all the other promises under discussion *are*. That is, that they are blanket mandates.
On the contrary, the immediate context is dealing strictly with dealing with a sinning brother within the Church, and the larger context of the rest of Scripture does put restrictions on selfish prayer. To take this one verse and try to isolate it so as to say it means that you can get anything at all you want just by finding someone to agree with you is as faceteous as reading Mt. 19:21 by itself and taking it as a general and absolute command to everyone.
The reason we don't in that latter case isn't because some outside authority says otherwise, but because the immediate context indicates that Jesus was dealing with a specific problem for that particular young man, and because the context of the rest of Scripture shows us many righteous men and women of means.
If you wanted to be a little more narrow about the context (isolating vv. 18-20), you could also understand the "two or more" clause as relating to the binding and loosing of spiritual powers in heaven, but this still isn't a blank check for earthly possessions--rather, it's a general rule about spiritual encounters. Those who have studied this area know it to be true that certain powerful spirits (like on the level of the rulers of cities, states, and nations) require prayer and warfare by many Christians to succumb, which once again is a reason why deliverance ministers work in teams rather than solo.
Which brings us back to the issue of authority over spirits. Jesus gave this authority first to the Twelve (Mt. 10:8), and then to the Seventy (Lk. 10:17-20), and then to all who believe in Him (Mk. 16:17). This makes it a universal gift to all the Church, not one limited to the Apostles or professional clergy--in fact, we see others, like Philip (Ac. 8:7), doing the same.
Now, given that universality of the gifting, is there anywhere in Scripture where the gift of authority over the spirits is limited? No. When Paul speaks of differing gifts that not all have equally in 1 Cor. 12-14, the gift of exorcising demons is notably absent. On the other hand, Eph. 6:12-18 makes it clear that every one of us is at war with "spiritual hosts of wickedness in the heavenly places," and that God has equipped all of us to fight this battle.
That is not to say that everyone is called to deliverance (or exorcism if you prefer) as a ministry--just as we are all called to exhort each other, but not all are called to be prophets, or how we are all called to disciple those we lead to Christ, but not all are called to become teachers. However, it does mean that a person in Christ has the authority over demons that Christ gave us in the aforementioned verses whether or not that's their calling.
If you disagree, I welcome you to build your case--preferably on something other than an inference made from a completely unrelated issue in Scripture. If you want to bring in external authorities, we can certainly do so. I have a fair library on this subject to support my position if it comes to it, as well as some personal experience to add.
I'm looking forward to your response, and it's always a pleasure to find another Berean. God bless.