Yes he can but he but he won't be able to trade them while he is still a f2p.
you can keep all your items but you can't use them in F2P.
You can craft air runes just south of the Grand Exchange in Varrock
No
Trading in F2P is simply exchanging items if that is what you are asking however th term "trade F2p" is something i have never heard of and i have played 3 yrs...
You can merch items.
No. In free worlds, members items cannot be used in any way, nor can they be sold. Your only options are to keep the items, or to drop them in order to get rid of them until you are able to sell them via the Grand Exchange. You shouldn't need to worry however, since members items occupy members' bank spaces and do not affect your free bank spaces.
a gold bar and an amulet mold will be needed but you can get one for free at lumbridge F2P
No, that is only for members. However, F2P players can BUY certain items from treasure trails - for example, Saradomin armour, or gold-trimmed armour.
Yes. As seen with several P2P players that go to, say, F2P Clan Wars, they retain their members skill capes, and they can still even do the emote for their members skill capes. Also, you can wear any members items in F2P worlds, although you do not get any bonuses from them. Once you take them off, too, then you cannot put them back on unless you go back to a P2P world.
No you can't use anything that is pay to play in nonmembers world.
The corrupt Dragon 2h is best tradable weapon on F2p, although is degrades rather fast. The Gravite 2h is not purchasable with gold, rather Dungeoneering tokens, and also degrades, but is the best weapon in Free-to-play.
The only Guthix items in RuneScape f2p is the Guthix rune armour. The only way you could get these items for free are via gift, drop party, or player killing.