It isn't difficult to do a backdoor Roth if you have an existing traditional IRA. It just creates tax implications that you need to be aware of and decide whether or not you want to put up with the taxes and record keeping.
1. One option, if it is available to you, is to roll your traditional IRA into your traditional 401(k). Your traditional 401(k) must accept incoming rollovers. Mine did. You'll have to check with your plan administrator and/or the plan documents to see if yours does. Another option is if your $1.5K has been contributed for 2017 or 2018, you could recharacterize it to a Roth IRA; this would give you a clean slate to do a backdoor Roth.
2. No. But you should keep track of them separately, because they are considered conversions, not contributions, and the tax treatment is different.
3. Depends on your goals and plans. Much ink has been spilled on this topic. There is some "investment order" page around somewhere that someone can link to that would have some advice. My generic advice would be if you're in a tax bracket under about 20-25%, then go with the Roth. If you're in a tax bracket above about 20-25%, then go with a traditional. But there are a lot of other things that could sway the analysis.
A backdoor Roth IRA, as I understand it, is contributing to a non-deductible traditional IRA and then immediately converting that to a Roth IRA. A mega-backdoor Roth IRA somehow involves contributing to an after-tax 401(k) and then rolling that to a Roth IRA. It's not clear which you intend to do based on your last sentence.