In the sense 1. that you provide, -less and without are indeed synonyms, and your example of sinless is one instance of that: sinless means without sin/ having no sin.
We find in the Bible an instance when "without sin" refers to any human being but with a negative meaning:
John 8:7 He that is without sin among you, let him first cast a stone at her.
It is interesting to note that sinless is not once used in the Bible, not even by modern translations like the NRS or RSV, possibly because "without sin" is considered more solemn. However, both expressions are commonly used and differ little in frequency.
I have found a few instances where the two are used together to explain each other, among which this one:
Even if through the omnipotent power of Divine grace the soul can truthfully avow that, keeping itself and kept by God, it is living an unsinning life, it can never say that it is sinless, or without sin: that it has no sin.
(The London Quarterly Review, William Lonsdale Watkinson, William Theophilus Davison, Volume 52, page 189)
As your second meaning states, the suffix -less can also mean:
Unable to act or be acted on in a specified way: dauntless. (AHD)
not doing; not affected by : tireless (Oxfordlearnersdictionaries)
For example, if you describe a person as selfless, it does not mean that the person does not have a self, but that it is not driven by self-love.
Even within the synonymy I pointed out in sense 1., the meaning of -less depends on the word it is added to, and also on established use. If you say the homeless, everybody will understand that you are referring to the status of people who typically live on the streets. But if you say people without a home, this is a less established phrase and may be interpreted as referring to people who were temporarily left without a home because of some natural catastrophe or mishap in their life.
Learn-english-today points out to another meaning -less can have:
The suffix -less added to an adjective means without or insufficient,
- fearless (without fear) or careless (insufficient attention).
CaGEL calls -less an affixal negator (p. 788), and records yet another meaning:
countless and fathomless have modal passive paraphrases:
- that can't be counted/fathomed. (p. 1711)