Mix brands of chainsaw chains

Both. Initially you may find that hand sharpening is kind of slow but once you get the hang of it, and realise you really only need to just slightly touch up each tooth, you can be back cutting in about the same amount of time as it takes to swap chains, or even less.

Always take a spare chain. Even if you touch up the working one rather than swap it out, there's always a possibility of damaging the chain, necessitating a replacement.

Back home it's worth looking into the cost of having the chain(s) professionally sharpened, as opposed to the time you're going to take to do a similar job. Also, you should be able to get a bunch of sharpenings for the price of even a mediocre machine sharpener. At least that's how the costing works out where I am.

I'm not familiar with that particular bar, so can't tell you anything about that spot. However, if you intend to drill anywhere on the bar a normal drill bit is not only going to fail, it's going to get damaged trying.

If a drill is your only option you need to get a masonry bit and regrind the point to about the same shape and angle as a regular bit. Use an Aluminium Oxide wheel for that. Drill at a slow speed without applying too much force, preferable using plenty of cutting oil because otherwise the tip can easily get hot enough to melt the brazing.

An alternative to drilling is to use a carbide burr in a pencil grinder.