Hey! Are you very familiar with the term "forcing moves"??
Tips for spotting/avoiding checkmates (besides doing puzzles)?
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Are those moves where your opponent *must* respond, because either their only legal moves are limited (check) or because they'd be at a severe disadvantage if they didn't?
Yes that is it! A simple way to list forcing moves would be:
1) Checks
2) Captures
3) Threats (to make a check or capture next move)
I wouldn't worry too much about number 3 just yet... but would try hard to find each check, and each capture, on every single move. It gets difficult when there are lots to look at, but like everything, it gets easier with practice. When you start seeing each check and capture your opponent has available, you won't overlook simple mates and won't hang material as often. 
Third match white had game in 2 after 40. White g3 black king did not have to move but could not take g3 because of Rook and King. Black would have moved a pawn. Then white Rf4#. Now because of time restraints this easy mate was missed.
Play slower time controls and really think before each move.
Games 1 and 2 I saw where you missed many safe checking opportunities but never did find the mate in 1 you wrote about.
Feb 7, 2021
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#8
You need to do even more puzzles. Even if there is not always a checkmate immediately, you'll at least know how to set it up. For this, solve mating net puzzles. Then if you keep training by solving very hard puzzles (like 8+ move combinations) then spotting a simple mate in 1 or 2 won't be a challenge. But if your puzzle level is mate in 1 or 2 then you'll have a harder time finding them in a real game
Feb 7, 2021
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#9
I personaly find puzzles not that instructive. My advice ist: There are so many articles and videos about this theme here on chess.com but also on youtube, just have a look and spend some time learning the basic mate pattern (the arabian mate, anastasia mate, backrankmate etc.). I for myself love the lectures, they are very instructive and well made. For example here: https://www.chess.com/lessons/finding-checkmate-1
Have fun and play on
Feb 7, 2021
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#10
If you see any moves at all that can check a king (yours, defensive, theirs, offensive) start looking for a tactic to use it or defend it.
In 2 of your games you got hit by the corner pocket, a queen takes the pawn next to the castled king with backup and its over. This is one of the 2 most common ways to lose out of nowhere. The second one is the back rank mate. Watch for both of them: if their queen can attack your castled king's pawns, guess what they are going to do soon? If your king is behind 3 of his own pawns and there are rooks left on the table, guess what?
back rank example:
Feb 7, 2021
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#11
in the last one, lets rewind to move 32. look at 32)g4 and see if you can mate the computer from there in a few moves? See how g4 blocks both h5 and f5 for the king to run to? That is the key for that game.
Using your first game as an example (ignoring all the inexperienced oversights).
11. Ng5, so white has a piece near your king and Qh5 is about to follow.
That is your "early" warning.
If you think about it, two pieces are needed to mate.
The best defence is to move your pieces towards your king.
11. ... Ne7-c6, well no moving away from your king, you might find a ...Ng6 helps later.
Clearly 11. ... Nb8-c6, rather complex but that knight might go d4 and f5
16. ... c4! was very good (Bb6->Kg1)
21. ... yes you are a piece down but all those passed pawns are winning.
Notice also that white has three pieces creeping your way.
Bearing in mind how a rook moves, consider the type of attack, you don't open the h-file.
After 23. g4 look what white has QR 2 pawns hitting your king.
When 23. ... Nxe5 (see Nb1) and 24. ... f5 create an escape for your king.
My last three games (all losses) the analysis shows I was in an overwhelmingly winning position, but I lose - blindsided by a checkmate out of nowhere or by running out of time frantically trying to mate someone as they dodge around pawns.
I've been doing puzzles a lot, but the mates in those puzzles are usually really straightforward - or when they're not, you *know* there's an answer, so you can spend time looking for it.
What are some good ways to be more aware of checkmate "availability"? I already play very slowly and the games I win are usually from being down 2 or more minutes, so "taking more time to check my opponents moves" isn't very useful for me, heh.
(I also do not enjoy time controls longer than 10 minutes, so while I know common beginner advice is "longer games", I would prefer to find other methods)
The Games
Lost because I freaked out and thought I'd missed an obvious pawn takes Knight capture for several moves; the position immediately collapsed afterwards. Obvious in hindsight, but not sure how to see in the moment.
Lost from +9.8 after blundering mate in 1. I'm used to finding this for myself but not for seeing if my opponent is lining it up, and I'm not sure what to do if I *do* see them lining it up. In the analysis, I was actually *extremely* surprised this was a winning position for white, if M1 is just around the corner.
This last one is just embarrassing and frustrating - up a queen and a knight, couldn't checkmate, blundered the queen frantically trying to figure out how to mate before I ran out of time. The computer moves are no help because they're not moves I'd ever realistically see - most of the setups for mate that I missed don't seem to deliver check.
Basically, I'm very comfortable fighting for good positions and I find that part of the game quite fun. Converting a winning position, even an overwhelming one, seems to be eluding me. Any advice would be great.