I am playing a solo Melee fighter. If I put one point into nature magic just for lesser healing, will my nature magic skill continue to rise as I heal myself? Will this have a negative effect on my melee combat skill?

If I'm not mistaken, your magic skills will only rise if you actively use attack spells, so no.
And unless you use spells that split EXP gain between masteries (the spell's description clearly states that, if not then the feature is not present in the spell), it won't impact melee skills at all.


I personally like to go with nature magic and melee, so my character is the party's healer, but also not defenseless if I run out of mana - that said, I like to have an army of pets and one companion for banter (pets grow as you feed them items and don't draw EXP for levels the way human party members do).
There is a skill in the nature mastery tree that lets you harvest mana potions and the ranger skill tree has a skill to let you harvest health potions from bushes with the appropriate color (blue for mana, red for health), so I like investing in those too (get ranged to 5 to access the potions skill and leave ranged alone after that since I don't use ranged weapons).
Though I'd have to check at what amount of points you get what kind of potions (small, normal, large or better).

P.S. If you enable the expansion Broken World with mods, you can access the Fist of Stone skills if you actively use nature magic and melee, but that means high investment in both skill trees - some passive bonuses are good such as healing spells being more effective and your character recovering from being unconscious faster if you have a pet or a companion to fight until your character recovers - not necessary, but a nice bonus for those of us that like that class combo.

Not saying you have to use any of this, just saying it exists, if you're interested. :)