Two Questions About MC and Placements

You are overweight because even with fortunate placements, and latent talent, you have not dug it up. So, once you put in effort to better yourself you should see nice improvements. It's not like you get nice natal chart and everything happens by itself, no, you have to do the things for the things to actualize. I hope that you reach your athletic potential, sounds good to have five to six good placements with fire emphasis. :D

1- Look at the ruling planet of the sign your MC lies on. Where is it, what sign is it in, what house is it in? What are the natures and affairs of this. What aspects are being made to this ruler of your midheaven? Look at the affairs of the planets aspecting the midheaven (if any) and particularly the ones which are tighter. What professions do these planets rule? You can check in Azazel's Astrology what type of careers different planets indicate. Which of these planets in your chart is most prominent regarding the Midheaven+its ruler? What is the rough balance in meaning of all of them together in the chart?

Aspects to the 10th house cusp itself also matter yeah, the tighter the more imposed. You can read what this might mean by checking the description of the planet IN the house and adding the emphasis of the aspect and sign the aspecting planet is in instead, to your understanding then mixing it in your overall judgement. Obviously the planet is not IN the house only aspecting the cusp, but you can still get something out of this. You should also do this with the planets aspecting the MC's ruling planet, along with the IC cusp and it's affairs as said planets should also aspect it the same way on the same degree as the MC cusp.

2- Fire planets being strong can also indicate laziness and tendency to comforts, particularly Jupiter which is also a planet correlated with fatness and excess. Mars being in a weaker position can also encourage this behaviour of your Fire.

It's not planets having an effect on you, the whole chart is you. It's different parts of your soul connecting with other parts and forming a unique meaning. It's moreso the outer planets of Uranus Neptune and Pluto that give an "effect" onto people.

Whatever Fire wants it takes the path of least resistance. The less Fire owns to give it the ability to get what it wants, it's going to get more active and stoked up. Jupiter is Fire getting what it wants so it appears you're issue belies in this planet and your own sense of action. Check where your Saturn is as it rules discipline and also is an indicator for career depending on the sign it is in and overall placement. If your career indications over your midheaven require the Saturnian discipline and don't have anything to do with any kind of intellectual writing job, or being a spiritual advisor or something like that, then you will need discipline. Discipline even helps with those types of careers too, but with the Fiery ones it is absolutely paramount.

Another thing you should look into regarding career is the 2nd house + its ruler which rules finances and how you relate to the material world and money, what type of things you desire to buy, and how you attract money. And also the 6th house + ruler, which rules your working patterns, details and how you structure your routines, how these affect your health, day to day responsibilities and things like that. The 2nd, 6th, and 10th house + rulers in a row are all the Money, Work and Career rulers. Everything else relates to other stuff. The 10th house is more relevant to your long term goals and path, whereas the 2nd house is what you want for yourself now, the 6th is the details of how you're going to be achieving and failing at either depending on your own determination to get them. For you these should be in the fire signs as well, and this would indicate that a lot of your inherent elemental natures dominate in what you mean to attain in life. Potentially the cusps of these houses might also reside in either Earth or Water signs adjacent to them which would change the meaning and make your material desires and fulfillments more on the Feminine lines.

But yeah my answer overall would be to look at the aspects on the planets ruling those 3 houses. Say for the 2nd house ruler is Mars being aspected by the Moon(which doesn't need to be in the 2nd house), you would look at the meaning of the Moon in the 2nd house and blend it with the nature of Mars. Then blend the Moon/Mars aspect in of itself, to the whole mix. As an example.

If one has Aries on the 10th house for example, and Mars is aspecting any planet, count said planet as being in the 10th house even if it's not. Same goes for every other house especially for the Angular ones, for the succedent and cadent it becomes more intricate natured than that, but still applies on the base level.