I've also been very curious as to the OP's question of alternate actors considered for each of the roles. From everything I had read - it seemed that BB / Paramount had decided early on they wanted Scott Bakula to play Archer, as I never heard a word about anyone else (though, if his contract negotiations went on for as long as it was reported they did, I'm sure they had a few others in mind or they'd have been screwed if Bakula declined the part.)
As far as Bakula as Archer, I feel strongly that it was the part, as written, that made the character lackluster. I have seen Bakula in plenty other things, live and film, and have never found him to be a poor actor, quite the contrary. Bakula found some semblance of a footing in season 4, but so much of the bs they had him spewing and the preachiness they engrained in the character was so bad it wouldn't have mattered if they would've had Shatner, Stewart, or frankly any non-trek A-list killer actor playing that part - much would've still been embarrassing and contrived. Man, I know acting is a job and the actors are there to do what the writers tell them to do, but geez - some of those Archer speeches were as cringeworthy as the theme, and ultimately, and unfortunately, became indelibly linked to Scott Bakula himself.
I wonder how it would have gone with Trinneer as the captain and Bakula as the engineer and keeping the role as the son of the person that developed the warp 5 design. It makes more sense that the son of the developer of the warp 5 engine would be an engineer committed to making it happen.
Although now it would be hard to visualize, if it would've been from day 1 - I can see Trinneer rising to the challenge. I do however think that Trinneer really shined as Trip and that it was a perfect casting job.
-
And last thing - why do the promos on Sci-Fi for ST:Enterprise say "Meet a future where... ... ... the captain gets all the ladies." - what ST:Enterprise are they referring to???? :-) I could see it if it said "the captain's good friend Trip gets...," "or the ship's engineer gets ...," etc. - but none of that would make much sense, marketing or otherwise - and it wouldn't relate back to Kirk, who even non-trek people know, did get all the ladies and was much more the captain they wanted than the Archer they got.
Rhodeschroma.