I preached on Mark 1:29-45 recently and spend some time discussing and reading about the tricky textual issue in v41.
The issue can be highlighted by a comparison of the NIV with the ESV:
40 A man with leprosy came to him and begged him on his knees, “If you are willing, you can make me clean.” 41 Jesus was indignant. He reached out his hand and touched the man. “I am willing,” he said. “Be clean!” 42Immediately the leprosy left him and he was cleansed.
40 And a leper came to him, imploring him, and kneeling said to him, “If you will, you can make me clean.” 41 Moved with pity, he stretched out his hand and touched him and said to him, “I will; be clean.” 42 And immediately the leprosy left him, and he was made clean.
Helpful little paragraph on parables from thr NET Bible note on Matthew 13:2
Though parables can contain a variety of figures of speech (cf. the remainder of chapter 13), many times they are simply stories that attempt to teach spiritual truth (which is unknown to the hearers) by using a comparison with something known to the hearers. In general, parables usually advance a single idea, though there may be many parts and characters in a single parable and subordinate ideas may expand the main idea further. The beauty of using the parable as a teaching device is that it draws the listener into the story, elicits an evaluation, and demands a response.
Some quotes from Wallace about the distinction of present/aorist imperatives (pages 716-18)
[the traditional view suggests that] the aorist in prohibitions meant do not start, while the present in prohibitions meant stop doing.
[McKay’s suggestion is that] the essential difference between the aorist and the imperfective is that the former urges an activity as whole action and the latter urges it as ongoing process.
[Wallace summarises] the basic force of the aorist in commands/prohibitions is that it views the action as a whole, while the basic force of the present in commands/prohibitions is that it views the action as ongoing process. This basic meaning may, of course, be shaped in a given context to fit, say, an ingressive idea for the aorist. Thus if the conditions are right, the aorist prohibition may well have the force of “Do not start.” This is an affected meaning or specific usage. But to call this the essential idea is not correct.
Εν is use by Jesus as ‘in the power of’. Although there is no explicit mention of power/ability, the verb εκβαλλω and the context of demon possession makes this sense fit. The idea is not so much power/authority as one of agency (see below).
See BDAG sv εν 6 agency.
Wallace has discussion of whether εν + dative can express personal agency or simply means/instrument. In this context of driving out demons this issue is heightened because it relates to who has authority over whom. Is Jesus the subject using Beelzebub as an agent or vice versa. Perhaps this is why Jesus reacts so harshly with talk of the ‘unforgivable sin’.