Hi barvin and welcome to this community.
Great that you are including the Berg in your tour of SA. Its a treasure that is often overlooked.
To answer your questions.
- Yes, Witsieshoek is the best, closest place to stay if you don't want to drive much in the morning. It is under new management and they are slowly getting it jacked up again (it was getting a bit old and run down). You can sleep at Sentinel Car Park but accommodation is very basic - just a dormitory with bunk beds (no electricity, no food and no bedding).
1. Yes, you most certainly do stand a chance of having beautiful, clear weather even in January. It just can't be predicted accurately. If, a day before your trip, the weather is misty and rainy and the weather report is not good, you may want to consider an alternate hike lower in the valleys.
2. For guides I would consider contacting
Sungubala,
Ian Shooter, or even Witsieshoek Lodge.
3. Hard to say exactly. But for a very relaxed pace I would consider 3 hours to the top, an hour to relax there and 1.5-2 hours back down.
4. Once you are at the top, the steep bits are pretty much over and it becomes a lot flatter, so you can cover distance quicker. Consider 1-2 hours one way to cross the Amphitheatre from Tugela Falls.
5. Might be worth it to have a real outdoor experience, especially if you have a guide organising all the gear and cooking etc. The security issues up there are more of a pest than they are dangerous - however you might spend a restless night listening for sounds around your tent, thinking it is a Basotho trying to steal your stuff. So consider that too, especially for your girlfriend, if you want her memories and experience to be good.
Places to stay after your hike: Cathedral Peak area is a personal favourite of mine, so consider Didima or Cathedral Peak Hotel (note: both places are pricey). You can also camp there. For a shorter drive after your hike (especially if you finish late) there are many places below the Amphitheatre.
Hope this helps. Please let us know how your plans go, and we always like hearing how it went afterward!