I'll try and answer all your questions, but I'll be brief. Since most are unrelated and you want more info for one or more of them, it might be better to start a new discussion/thread for each subject. It is really difficult to keep things on topic with several different subjects being discussed in a single thread.

What does it mean to be confessional vs non-confessional?
Confessional: To embrace a Confession as one's own as representative of one's own beliefs.
Unconfessional: Actually, this is an impossibility.

The claim is that a person does not recognize any of the historical written creeds as being sufficient/worthy of their approval. In short, the person is a "Rogue"; not identifying with any group or denomination. Typically, the reason is because the person holds to a doctrine(s) which the formal Confessions reject as heretical. Now, I said it is an impossibility for someone to profess to be unconfessional. Why? Because as soon as you ask that person what they believe about xxxxxx and they answer, they are stating THEIR confession. So everyone in the general sense is confessional.
What good does it do to baptize an infant? I know countless people (even in my own extended family) who were all baptized as infants in the Presbyterian church in Northern Ireland, and not a one of them served God as adults. So, I find it hard to wrap my head around that one and I would like to understand why paedo over credo?
What good does it do to baptize an adult? It isn't what baptism does to an individual but rather what baptism MEANS! Credobaptists for the most part (exceptions noted) define baptism subjectively, e.g,
Baptism is an outward sign of an inward reality. Paedobaptists define baptism objectively, e.g.,
Baptism is the visual demonstration (sign) of the Gospel; God's salvation in Christ. As the water washes away the filth from the body, so does Christ's blood wash away sin. It is also a (seal), i.e., what the sacrament of baptism visually demonstrates, it is likewise applied to the recipient who has true living faith and is thus in union with Christ.
To say that Baptists ONLY baptize believers is patently untrue, for it is virtually impossible to infallibly know who has true faith. In fact, the honest Baptist will admit that not everyone who is baptized is saved. This is one of the main reasons for the change in name from "Believers Baptism" to "Credo Baptism". For the latter better expresses the reality of the situation; Baptists baptize someone on the basis of a "credo", a PROFESSION of faith vs. actually possessing faith.
I believe 100% in the doctrines of grace, Calvinist Soteriology, and in credo baptism. I no longer believe in dispensational Premill. Leaning heavily toward Historic Amil (based on what I've been learning over the past few weeks only) although I haven not ruled out Historic Premil either. Would a Reformed Baptist church be a good fit for me? Are there any other options? I don't have a RBC church near where I live.
At this point, a Calvinistic Baptist church would suit you well, i.e., a Baptist church which not only holds to the infamouse "Five Points of Calvinism" (soteriology), but also believes in the pure preaching of the Word, administers proper church discipline for violations of doctrine and/or life, and practices biblical worship, aka: Regulative Principle of Worship. There are such churches in the USA, albeit few in comparison to all Baptist churches. The same could be said for paedobaptist churches as well; few vs. many.
What is meant by the Regulative Principle of Worship?
In its simplest form: The proper worship of God is that which God has expressly commanded in Scripture. There is what I consider an extreme view of the RP which believes that only Psalms can be sung (Exclusive Psalmody) and sometimes no instrumentation (Acapella singing only). There are some excellent articles on this subject on The Highway in the "Ecclesiology" section: Calvinism and the Reformed Faith > Ecclesiology > Worship (heading).
Lastly, HP and HA are so distinctly different, so how can they both be considered Orthodox?
That depends on who you ask!

IF one is asking if HP fits into the main frame of Christianity, then the answer could be yes, for HP doesn't really impact on the foundational doctrines of the faith, e.g., Trinity, deity of Christ, historic fiat creation, virgin birth, death, burial and resurrection of Christ, etc., etc. However, if the question is asked in reference to eschatology specifically, then things get rather sticky. Historic Premil, IMHO, is an admixture of Amillennialism and Dispensational Premillennialism. HP is held mostly by Baptists, but there are two denominations which are Presbyterian and confessionally hold to HP. In the US it would be the Bible Presbyterian Church and in Europe the Reformed Presbyterian Church (there are some churches in the US and Canada too, one of which I attended). Both those denominational founders were close friends and both were friends with Bob Jones. That is one of the major reasons for their similarity, especially in regard to their Fundamentalist beliefs.