Sorry if my reply is all over the place, feelin off today.
It's the opposite, I think you interface DOES allow you to direct monitor, so even if pro-tools does not allow it then you will be good to go still. Input monitoring is just so you can listen through your headphones your own performance while you are recording. For me it's vital because you will also hear if your microphone is in a bad position, and you would use your headphones to see the effect that your mic positioning is having on the sound. That's all it is. And if you are singing and playing guitar at the same time you probably wont even bother with a metronome (that click). I will always use it because otherwise I will struggle to keep all of my tracks in time, especially if the intro of a song has no drums for me to follow along to and I want to time a hard hitting strum so everything comes in perfect timing together. I normally use a metronome for scratch tracks then I program simple drums in and then use that in replace of a metronome. but if you're playing guitar+singing then I think honestly a metronome could actually hold you back and I wouldn't use it. (using a metronome could make your performance sound robotic, and destroy any natural response or urge to play at a faster bpm during a particular part of a song that might sound nice; Chorus?)
It seems like you are doing fine, if you already had great success in the past with no plugins, or metronome then I wouldn't worry. A plugin is something you would place into an insert (a spare slot) in your track so you can further process it if you need to. If you wanted to EQ, maybe you mik'd up too close and want to turn down the bass a bit you load an EQ and turn down the bass part of your guitar track, there are hundreds maybe thousands of different plugins. You want reverb? you use a reverb plugin. Delay? etc. etc. All DAWS have a very good selection of free plugins, and you can get professional mixes using them. I can't speak for ALL daws, like mixcraft and audacity which I believe are the free ones but any pro DAW is going to have everything you would ever really need straight out of the box.
The only reason I said about using plugins is because I thought you may have been doing actual mixes with many tracks. I said I personally need at least the ability to load in at least 5 minimum (for my purposes). Really great job though on getting away without using them before in the past. I would never be able to do that. I am not a great performer, and using cheap budget harsh instruments so I need to use a compressor plugin to control my loud and soft parts of the guitar, and an EQ plugin to try and make the guitar sound better than it really is, and saturation plugin to make it sound like I recorded with a nicer tube pre-amp. And that is before I even start mixing for real. I would use reverbs and delay plugins also. But I don't think you should necessarily, your style of music might not call for it. it probably doesn't even call for a metronome to keep that strict timing, keep performance natural I reckon
If your computer can record fine with no pops and crackles then you are good to go. You may need to go into settings in protools and just adjust the buffer size to maximum (google how to do this, it's a very common process to set it as fast as you can get away with when recording so you have minimal lag, and when you have finished recording you set it all the way slow to give your computer some extra time to process your mix in playback when running through a ton of processing) which will help a lot if you are getting horrible artifacts where your computer is struggling to keep up, and seeing as you will not be monitoring (through protools) anyway you won't notice the lag.. If you are overdubbing tracks (recording just a guitar, and then recording just a vocal while listening back to the previously recorded guitar track, instead of recording guitar + vocals in 1 pass) just move your tracks back on the beat if you need to. If you find you are getting the crackles during recording which will destroy the sound of your track while you are on maximum buffer size, then your computer is probably not up to the job. I had a laptop of similar specs and I remember struggling getting a clean recording once I would get the odd POP during my takes (My audio interface was really crap though, you have far better than what I start out on), but it was getting a bit knackered which probably had more to do with it. You can also just try lowering the sample rate from 96, to 44.1khz to help. Once you have got your recordings you probably haven't got much else to worry about. Your Scarlett Interface will do most of the heavy lifting anyway. If only an odd pop, or crackle you may be able to edit it out, if you are overdubbing tracks it's way easier. When I record just a guitar part I normally record more than 1 take, that way if I did get a pop, or a car or a dog barks and I didn't notice it, then I can just cut from the decent guitar take and splice in, or if I make a mistake during the middle of the song I will probably keep the take if it's generally a good take but do another take and carefully edit out mistake using the good take. It's way easier for me to do this though as I keep strict timing with a metronome. (Also if I wanted to get that nice stereo effect then I have a spare take to play with by panning both hard left and right) Sometimes I am very undecided on if I will need it, regardless I don't think it's a bad idea to record more than 1 take, it's actually saved my ass on more than 1 occasion)
Can's is slang for Headphones. sorry.
I have a lot of homework to catch up on still, most on here are faaar more experienced than me and really know their stuff. And when I say "most" that is not an exaggeration at all, I am still a newbie.
Now I'm really not too sure on this but that mbox back in 2007 could possibly had been recording in 16bit? I know a lot of cheaper interfaces back then were like this I tried to look up the specs but don't know what version you had but not too important anyway you will now be recording in 24bit which brings the noise floor down even further and you do not have to record so hot. You will probably have the option to record in 192khz aswel but...... really I think that's overkill. As you said, will be dithered down anyways when mastered.
You are doing things right... great infact, especially if mastering engineer said he didn't have to do much for your tracks. Do it again!
I really don't mind trying to help out, may not be able to get back very quickly sometimes but it's definitely no sweat.
EDIT: Mixcraft is not actually a free DAW my mistake
I hope this time I didn't confuse you with any slang or technical talk, I tried to take it a bit slower in my ignorance I sometimes forget I'm talking to somebody who's just startin out on the newer technology.