Venue by default is the defendant's (your) home court unless you prefer to use a different court (e.g. if you are working away from home).
DCBL seem to like to request the Claimant's home court, without specifying which court that is, or giving any reason for not using the defendant's home court, despite knowing that this is the default and that they would need a very good reason to get it changed. This is not something that you need to be concerned about, as the courts will simply ignore it.
Less weight will generally be given to evidence not given in person - but if your evidence does not rely on your credibility, that is not an issue.
It is potentially easier to get your point across in a live hearing, but it is generally easier in general to do the whole thing on the papers as regards wasting half a day in court. If you are concerned that you might not have covered all the bases, in a live hearing you should be able to expect the judge to address anything that is obviously missing, potentially giving you an opportunity to fill in the gaps - although that does not generaly extend to raising any new defence points or presenting evidence not disclosed before the hearing.
The courts tend to be very petty regarding the DQs - not so much as regards how accurately you fill it in, but despite the answers usually being the 'default' answers, will quite happily dismiss your defence if you don't submit an N180.
As regards unavailable dates, list any for the next 12 months - if the courts are busy/backlogged/incompetent, they will quite happily list a hearing for a date outwith the 6 month period they asked about and simply tell you that if you want to change it you need to apply with the correct form and correct fee (neither of which are specified beyond that it must be the correct one).
The general consensus is that mediation is a waste of time if there is no "middle ground", and that it generally consists of the claimant trying to browbeat the defendant into giving up by telling him that his defence is hopeless.
If you want advice specific to your case, as opposed to general advice regarding the N180, we are going to need something to work with. If you already have your defence sorted and just want to check the specifics of the DQ form, not so much.