It may sound screwy, Shermy, but if only half the information is accurate, it doesn't sound good.
I live across the channel from Oak Harbor on Camano Island and look directly out at the town. The base is only a few miles north of it. I was a visitor on the base during Desert Storm and it's a very large airbase.
As to your question of why a terrorist would have both explosives, there are any number of answers for that: TNT is block high explosive while RDX, just as you thought, is the principle ingredient in plastic explosives. They could both be used for the same purposes (except TNT can't be molded). RDX is also used in some det cords, which is used to connect separate charges to detonate nearly simultaneously and is also used as an ingredient in some blasting caps.
Given all the above, if those arrested were palestinian or Arab Israelis, it adds up to a highly improbable set of coincidences, if there nothing to this.
The base could be a target, but there are other potential targets as well in the area.
Oh, almost forgot. It is very easy to speed through Oak Harbor and is known locally as a speed trap.