Can one really run a web development business alone at least short-term?

I hear you. Designing is not my specialty either. I know the basics and can design above average, but I know my weaknesses and strengths. I've found a few ways around this. 1) I've partnered with a designer who is great in web design. I challenge her to come up with ideas that make me sweat as a developer. Makes me have to learn new techniques. This allowed me to instantly double what I charge for sites, since now I know that a site I create is worth it. Before I felt bad handing clients a well developed but ok designed site and charging a lot for it.

2) Working with agencies. I found that I really enjoy working with small marketing firms and local print shops (lack of a better word) that want to offer better web stuff to their clients. They handle all the dealings with the client and tell me what they want the site to look like and I make it happen. Often they give me psds or marketing materials they've already created for the client that I can pull from to make the site.

3) Become a master of developing premade code templates. Not sure how else to say it. For me this is Wordpress. Anybody can pay $80 or whatever it is and have access to tons of great Wordpress themes, but not everyone can figure out the code structure of Elegeant Themes' framework and be able to go in and break the 'premade' themese to your will and output exactly what the client needs. I love doing this because of the low overhead, but I know I'm starting out with a pretty well designed theme that I can just manipulate to the client's needs and will not act like some premade theme out of the box. I use a lot of Elegant Themes and WooThemes for this type work (don't judge me :) )Clients don't care what you started with, only what you end up with.

Your other point. Job experience never hurts. You may find that you like working in an office more than freelancing it. I wish I had that experience and still consider every once in a while. If you end up getting a ton of side jobs and feel like you'd be better off on your own, then you can take off and go back to the freelance work, maybe even continuing to work contract for the company you were working with. Most companies like their employees to work on the side as it encourages them to learn new things and stay on top of technology--- as long as it doesn't interfare with their job duties.

If you want to build up a portfolio, starting writing code and posting to github. If someone is hiring a developer then they aren't expecting chocolate happiness all over the design of an app or webpage. They are expecting performance and proper code structure. Go online and look at jobs that companies are asking for help with on freelance sites and start writing code to solve those problems, even if you aren't getting paid for it. Post projects that you are working on on gitHub or a blog and start building some klout.

hope that helps.