Pros Cons - iMac vs MBP (used as a desktop)

New Apple convert here. I've been searching forums for days but haven't yet found up-to-date answers to my questions. If you have some time, please continue reading! :-)

I just bougt the new iPad Air--yay! It'll probably never leave my side.

I also just bought a MBP retina, late 2013, 8GB ram, 2.4 GHz i5, 256 flash storage to replace our dying Dell desktop.

My background and estimated usage: I'm a homeschooling mom and also a writer/blogger. I am a huge content consumer and just getting into more content creation as I learn more about video and photo editing and web design. Not a professional photographer or videographer by any means, but a heavy Internet user with strong curiousity about everything a Mac is capable of. And don't forget I'll be relying heavily on our computers as we homeschool our children.

That all being said, here's my dilemma:

Should I buy a maxed out iMac to future proof for our family for a long as possible OR should I keep the MBP with mediocre specs and buy a cheap external display so that I can have that feeling of a desktop with more screen real estate that I already miss?

Some notes:

Yes, I realize the main difference is portability and that an iMac could be configured to have better specs for less $$ than a laptop.

I love the idea of portability, but in reality I feel like if I had an external display I'd mainly use the laptop as a desktop. (I originally bought a Thunderbolt display but returned because it was very expensive and I anticipate an upgrade from Apple in the future.)

I also tend to overestimate my future usage. I could spend the next 5 years just checking Facebook, or I really could get into graphic design and video editing, so I'm not sure if I should just go entry-level or if I should max out the specs to future proof our investment. Money is a huge factor, but if we're going to do this, we want to do it right.

Questions about iMac vs using a MBP as a desktop:

1.Since having my MBP for the last few weeks, I feel like I'm charging it constantly. At least twice a day. Is this normal? And is it a reason to get an iMac?

2. Will using a MBP so heavily and hooked up to an external display for long periods of time have any negative effect on its battery or life or its screen since the MBP lid will be closed and the screen subjected to more heat?

3. Suggestions for a decent but inexpensive monitor to use if we end up keeping the MBP?

4. Any other reasons I haven't considered as to whether a desktop or a laptop + display would be better for my situation?
5. When plugged into an external display, does the MBP run off the battery or does it switch to AC power without affecting the battery?

6. For my suggested use, should I keep the somewhat baseline specs of the MBP that I already have, or should I upgrade everything (just in case)?

7. Or, should I get the best iMac I can afford and if I ever feel like I really need a laptop, I can always buy a cheap one or save for another MBP?

As long as there are no real cons to using the MBP with an external display and that it can take heavy but basic usage day in and out without shortening it's life, I'm leaning toward keeping the MBP and buying a cheap external display until Apple updates the Thunderbolt display. I just feel like if I ever *want* to work in another room or outside the house, I'm going to appreciate having a laptop that functions as both laptop and desktop.

I'd appreciate any feedback. I know this was ridiculously long, but I need help from those who know! Thank you, thank you, thank you!