Are my prayers and fasting invalid due to the emphasis or alteration of letters in prayer, and inhaling steam or smoke during fasting against my will, and what should I do to avoid that?

Giving in to whisperings (waswas) and leaving oneself vulnerable to Satan is what led to this state. Waswas is a deadly disease. It is obligatory to completely and utterly disregard the whisperings and proceed with acts of worship. If you are struggling with the prayer, Allah does not burden a soul beyond its capacity, and a linguistic error (lahn) in something other than Al-Fatihah does not invalidate the prayer. Perform the prayer once, striving to establish it correctly. If you have exerted your utmost effort, then you have fulfilled your obligation. As for the matter of fasting, things that are difficult to guard against, such as road dust and rising smoke, do not invalidate the fast due to the widespread nature of the affliction (umum al-balwa) and the difficulty of avoidance. Therefore, you are not required to hold your breath, and your fast is not invalidated by such things.