1/15 - わけだ would make sense if there were reasons for and leading up to it.
人は年を取ると忘れっぽくなる。
高橋は年を取っている。
従って、高橋は忘れっぽくなるわけだ。
That's probably too straightforward, but yeah. I've also heard it as like "so you're saying that ... ???" Like drawing conclusions out of what people say. So no, I'd say that if it's just that one sentence, ものだ is probably the better fit.
1/19 - まいか is like a 'purposeful negative.' ものか is like a rhetorical question, so like "Do you really think he's gonna make the 6 o'clock meetup time???"
まいか is similar in this case, because you could say "Dude, do you really think I'd miss the 6:00 meetup time??" but I'm pretty sure it requires some personal effort. It's that 'volition to not do" something. Which yeah, in this case, it's talking about someone else, so you can't really speak for their volition.
1/26 - I wouldn't say 上は is "moreover." I think you're thinking of 上に, 上には. But I'm not really 100% on my 上's.
I think 上は is like "Since" or "Considering [previous]" or "Now that"(edict

)
弁護士になると決めた上は、苦しくてもがんばらなければならない
"Since I decided to become a lawyer, I've gotta がんばる even if it's 苦しい"
友人が困っているのを知った上は、黙って見ていられない
"Now that I found out my friend was troubled, I can't just remain silent and watching"
3/6. You're probably thinking of べきではない as like "shouldn't do," because it's べき+じゃない, right? It's more like "there's no requirement to" watch TV, as opposed to "there is a requirement to not watch TV." In English "not supposed to" and "supposed to not do" are pretty similar, but here they're different.
どころではない is kind of more like "This isn't the time to watch TV!!" thing.
Edited: 2010-11-01, 10:19 am