Can ultimate paramatha reality ever be observed or realized with only samatha concentration meditation on a concept?
The word 'samatha' ('tranquility') is not synonymous with 'samadhi' ('concentration').
Regardless, both 'samatha' ('tranquility') &^ 'vipassana' ('insight') occur in tandem when right samadhi ('concentration') is developed (refer to MN 149).
If fact, the ultimate paramatha reality cannot be observed or realized without samatha (tranquil) concentration (samadhi) meditation.
you have to mix your memory of right view, sati, right investigation with the jhanas and the buddha calls this right samadhi and that's the basis for ''seeing things as they really are'', ie conditioned things as anatta, anicca, dukkha and the ''stilling of all formations'' as ''peaceful'' and so on... Currently, nobody knows how to mix sati and the jhanas and even the jhanas alone are already hard, jhanas as the ''pleasant abiding in the present life'' like the buddha calls them when there is not much sati with them.