(1) Intuitively, this seems very likely. Textual support could be found in the fact that the seventh Factor of Enlightenment and the 4th BV are basically the same, upekha.
(2) This has indeed been my personal experience. Textual support could come from the opening lines of the Karayana Metta Sutta which suggests that when one is already skilled in goodness is the time to start cultivating meta, the first BV. Of course, dana precedes sila.
(3) This logic seems dodgy to me 
because the statement isnât clear about whether âPerfectâ or âPartialâ is being considered. âYesâ to âPerfect abiding in the BVs requiring perfect silaâ. But possibly imperfect or partial sila, or temporary removal of taints etc can provide a basis for some temporary level of BVs. People at different stages along the path are able to let go of bad stuff in part or temporarily. I assume that it is to such people that the Buddha sometimes gave advice, eg to the mendicants he addressed in MN21:
Mendicants, there are these five ways in which others might criticize you. Their speech may be timely or untimely, true or false, gentle or harsh, beneficial or harmful, from a heart of love or from secret hate. When others criticize you, they may do so in any of these ways. If that happens, you should train like this: âOur minds will remain unaffected. We will blurt out no bad words. We will remain full of compassion, with a heart of love and no secret hate. We will meditate spreading a heart of love to that person. And with them as a basis, we will meditate spreading a heart full of love to everyone in the worldâabundant, expansive, limitless, free of enmity and ill will.â Thatâs how you should train.
This is perhaps the source of why
The Buddha didnât just say, âDevelop your sila and watch the BVs flow inâ; he instructed his followers to work on their defilements, to train and to spread love all around; hereâs another eg:
Reverends, the ascetic Gotama teaches his disciples like this: âCome, mendicants, give up these five hindrances, corruptions of the heart that weaken wisdom, and meditate spreading a heart full of love to one direction, and to the second, and to the third, and to the fourth. In the same way above, below, across, everywhere, all around, spread a heart full of love to the whole worldâabundant, expansive, limitless, free of enmity and ill will. Meditate spreading a heart full of compassion to one direction, and to the second, and to the third, and to the fourth. In the same way above, below, across, everywhere, all around, spread a heart full of compassion to the whole worldâabundant, expansive, limitless, free of enmity and ill will. Meditate spreading a heart full of rejoicing to one direction, and to the second, and to the third, and to the fourth. In the same way above, below, across, everywhere, all around, spread a heart full of rejoicing to the whole worldâabundant, expansive, limitless, free of enmity and ill will. Meditate spreading a heart full of equanimity to one direction, and to the second, and to the third, and to the fourth. In the same way above, below, across, everywhere, all around, they spread a heart full of equanimity to the whole worldâabundant, expansive, limitless, free of enmity and ill will.â
Ven AnÄlayo in Compassion and Emptiness in Early Buddhist Meditation p25 says that use of the stages of cultivating metta for self-revered person-loved person-neutral person-difficult person-enemy is not found at all in the EBTs.
This comes from the Visuddhimarga chapter 10, 8ff and has been much adopted subsequently, in Burma, by Ajahn Maha Chatchai in Thailand and by our very own Bhante @Sujato, who has taught a number of retreats on how to cultivate metta.
Bhante S makes it clear in the introduction to the 2005 retreat that he is teaching the Visuddhimarga formula and explains why he thinks it is valuable for non-enlightened people to cultivate metta.
EBT purists sometimes argue like this:
I mean no disrrespect to Ven Thaniyo when I say that people report (myself amongst them) that doing metta practice, either by radiation as the Buddha and AnÄlayo suggest or by mantra following the Visuddhimarga, has a noticeable immediate effect: at least when it is combined with efforts towards purification; it might not work (if sila is being seriously neglected). My personal observation is that if someone is irritating me, systematic generation of metta to them achieves a positive improvement in their behaviour! I guess because it makes me kinder to them and consequently they feel kinder to me, act more pleasantly and less irritatingly. My sila is purified just a tiny bit. And theirs is too maybe. Such experiences make me think of the story behind the Karayanamettasutta:
According to post-canonical Sutta NipÄta commentary, the background story for the MettÄ Sutta is that a group of monks were frightened by the sprites in the forest where the Buddha had sent them to meditate. When the monks sought the Buddhaâs aid in dealing with the sprites, the Buddha taught the monks the MettÄ Sutta as an antidote for their fear. The monks recited the sutta and felt better. Their good cheer then happened to quiet the sprites as well.[5][6]
In sum, it would seem that the deliberate practice of metta in daily life and on the cushion can help push the development of sila. Temporary cleansing of the defilements can lead to temporary joyous experience of the BVs, just as permanent removal of the defilesments can lead to permanent divine abiding.