your marth is not too shabby - shibby
have you tried to abuse this technique? If done correctly you can take fox from 0 to death. Or at least to the edge of the stage, were you can then edgegaurd(his best ability!! use it and abuse it). Being chain grabed is extremely frustrating and does an amazing job at putting your opponent on tilt(plays recklessly, attacks blindly, ie, easy sheild grab/counter tilt). If you wanna practis CGing go into training mode and set the computer to flee/retreat and chase him down. They will DI properly after each throw so it kinda mimics a real opponent. The idea is that you chain grab until Fox's life gets around the 50-60% range, were after an up-throw he will be DIing right into a f-smash tipper.
Work on calming down and waiting for the right moment to attack, this is an important technique for marth as most cases if you miss a swing you will pay for it in some way or another, weither its a missed edgegaurd technique, punished smash after a whiffed f-smash, etc. Marth's range gives him a big up on a defensive/reactive style of play. It sounds like you already know the problem but cant get around it. There is always a way around something its just a matter of knowing what to do. For example:
You mentioned he spams lazer waits for you jump at him with an attack, then he rolls behind you and attack you in your lag. Well you have already analyzed the situation, now all you have to do is adapt to this. You have a few options, you can attack him with a f-air and properly L-cancel it. If he tries to smash you in your lag all he will get is your shield (if L-canceled properly, and continue to hold your shield). Then you can shield grab or roll away. Also if you notice your opponent being very roll happy (ie, rolls everytime u approach with an attack and gets you in your lag) you can just fake them out and do an empty shorty hop(a jump without an attack, so you have no lag to worry about) and counter them. Remember Marth is all about his range, and 90% of the time he has a move that (if timed properly) can counter most attacks.
I like to use L for my Lcanceling, I use L only for canceling, and R for sheilding and Wavedashing. Fox trotting isnt much of big deal, all it is that fox(I think marth can do it too, but not as well) can do a double dash in the same direction. Its really just a dash out of your first dash animation, I think its just a glitch in the game. It gives him a momentary boost (but only brief) in motion one way. Dash dancing is different in that you are just dashing back and forth to mess wiht your opponents head.
You should check out some videos, you will get all sorts of help from them. You will see that if you can abuse the chain grabbing and edgegaurding Fox really doesnt have anything on Marth.
In response to a fox that will rush and shine what u want to do is try to stay away from that situation first of all. Marth's f-tilt is very useful in this situation due to its range and speed usually u can catch a fox before he can get close to you and keep him away. when a fox actually gets in close which is sometimes unavoidable(drill shine perhaps) u want to watch out about going for a shield grab because of the shines almost lack of lag due to its 1st frame attack. when he moves in try to wavedash back and use and fsmash alot of the time this will result in a tipper or atleast a good smash sending the fox flying. if the fox happens to miss the shine u can try to shield grab him and being a chain grab as explained earlier in vol's post. another option in the crouch and poke method. which you use the dtilt to keep the fox away from you until you are lined up for either an fsmash or an ftilt. those are just a few options but u can basicly do whatever works best for you.
in regards to the uair juggle thats easy to answer and even easier to execute. when fox throws you up simply DI to either the left and away or right and away(try to mix it up to throw him off) of course this won't always work but its the best thing i can think of to do in the situation. usually they won't hit dead on so u won't go flying up through the cieling of the board. try not to use counter, as you get to a point where you are playing someone who is pro // semi pro or even just has a clue how to play smash counter is more of a burden then a blessing. the lag time on the move will kick the crap out of you so watch out because a missed counter means a painful punishment.
