I'm quite certain that a Skyranger that can't fit one more soldier isn't going to have a harder time bringing back 50 aliens and 2 supply ships then it will have bringing back 25 aliens and 1 supply ship. There is no lore reason I can see for not going to the next mission site.
As for healing - if you are able to survive the 18 hour flight back to base at all, then you're not going to bleed out. As for the skyranger having non-portable medkits: All military transports have full medical kits, and all soldiers in combat have first aid kits. This has been true for at least a hundred years. However, in X-COM portable bandages are a groundbreaking technology prototyped in 1999 by an international crack team of scientists in a secret underground laboratory that specialized in ray-guns, psionics, cold fusion and non terrestrial alloys.
Any decisions about whether you should be able to go from site to site should be about game mechanics, not medikit lore. At least until you start with medikits and can purchase more, and they fit in one tile instead of two.